LibJS: Make int_part a double in StringPrototype::to_string

u64 is not big enough to hold extremely large numbers, such as
4.192938423e+54. This would cause an integer underflow on the radix
index when performing something like `toString(36)` and thus cause an
OOB Array read.

Userland/Libraries/LibJS/Runtime/NumberPrototype.cpp

@@ -459,7 +459,7 @@ JS_DEFINE_NATIVE_FUNCTION(NumberPrototype::to_string)
     if (negative)
         number *= -1;
 
-    u64 int_part = floor(number);
+    double int_part = floor(number);
     double decimal_part = number - int_part;
 
     int radix = (int)radix_mv;
@@ -469,8 +469,9 @@ JS_DEFINE_NATIVE_FUNCTION(NumberPrototype::to_string)
         backwards_characters.append('0');
     } else {
         while (int_part > 0) {
-            backwards_characters.append(digits[int_part % radix]);
+            backwards_characters.append(digits[floor(fmod(int_part, radix))]);
             int_part /= radix;
+            int_part = floor(int_part);
         }
     }
 

Userland/Libraries/LibJS/Tests/builtins/Number/Number.prototype.toString.js

@@ -70,6 +70,62 @@ describe("correct behavior", () => {
         expect((30).toString(10.1)).toBe("30");
         expect((30).toString(10.9)).toBe("30");
     });
+
+    test("extremely large number with radix", () => {
+        // NOTE: The answers change depending on the engine, so the important thing here is that it doesn't crash
+        //       and is good enough.
+        // Example number generated from an ID generator in Raygun4js: https://github.com/MindscapeHQ/raygun4js/blob/777ba6e3cdede42011aa0b7f019473de6380009b/src/raygun.rum/index.js#L1204
+        const number = 2.2171010912173817e51;
+
+        [
+            [
+                2,
+                "101111011010000000010001011010101110000101111100111010000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000",
+            ],
+            [
+                3,
+                "122200222200200002020000221212111212020212021222122122201112112001202200202210122011020121220102211210120110",
+            ],
+            [
+                4,
+                "11323100002023111300233213100000000000000000000000000000000000000000000000000000000000",
+            ],
+            [5, "20213333043100433442123113244334300004432332001224240433304410013422100304"],
+            [6, "550003524220341511224004404200404220224024020444420402442220220420"],
+            [7, "4235625635040541506562064422461122351652161506062250163366335"],
+            [8, "573200213256057472000000000000000000000000000000000000000"],
+            [9, "580880602200855367001288271433120018026801542154811843"],
+            [10, "2.2171010912173817e+51"],
+            [11, "20941975227252065997040823726473715831623804aa6726"],
+            [12, "426276185347a25840488044080484880408844840480440"],
+            [13, "136bc76718505c84cb00730624426c569759715872b8b70"],
+            [14, "835c3959100c338282668c622a8a060a24602aa6a66cc"],
+            [15, "5e1ae5933690a58aad6a9546513bbbd49305ec050549"],
+            [16, "5ed008b570be7400000000000000000000000000000"],
+            [17, "7f3da30c676ag43gc65022d17gae829c8790d1d479"],
+            [18, "dba188ded00ha8a6c2800e808c20aagggg4g4822c"],
+            [19, "1af846i1f7ge0bfb7b4c3f1abd5a70g3bhd592fb6"],
+            [20, "40d316h12f73ec848404c8c0g4400gc48c400804"],
+            [21, "cd59ci682hih768393c4d1kc5757jkgg918b2dc"],
+            [22, "239f104idd3dikgg6e4eccg66aee26a0kc2kac6"],
+            [23, "93f4b0317667h2gm0gka6ia4h9emj7b6aef4ig"],
+            [24, "1lca597424748g080gggg80g08gg8880880g80"],
+            [25, "abii4g0njm70ejj3c39lajgi5l3bdf6gl25ne"],
+            [26, "2e8foaj1lec80c86cck264caii282m6m8e6a0"],
+            [27, "hiqii260pnbeqe894nd351nqpo5nm3j45jj3"],
+            [28, "4qmgqk0rk788oog4c8k0k0cg8o04cocckckc"],
+            [29, "1d2nmd4bln2jmf0k72sl2ejhmj7m8rq70flk"],
+            [30, "d8onrgfc6kaik26a4oqmee4ic0ksi0k0k2o"],
+            [31, "4b4u5lb8fb8ttacfifj5g1iapimkppcehrq"],
+            [32, "1fd025le2v7800000000000000000000000"],
+            [33, "h5mgn3kr83kucw13c6102w7n5loohbfct6"],
+            [34, "6e0244vn6ba0gw8ucgkmuwgk4oi02g8icq"],
+            [35, "2g7ohyr3v1t7c0jrq621aw56kflrj71pfj"],
+            [36, "z03wqcmb7dw00g80wow0sksk484wgswsc"],
+        ].forEach(([radix, result]) => {
+            expect(number.toString(radix)).toBe(result);
+        });
+    });
 });
 
 describe("errors", () => {