LibRegex: Bail parsing class set characters upon early EOF

Otherwise, we reach a skip() invocation at the end of this function,
which crashes due to EOF. Caught by test262.

Tests/LibRegex/Regex.cpp

@@ -779,6 +779,23 @@ TEST_CASE(ECMA262_unicode_match)
     }
 }
 
+TEST_CASE(ECMA262_unicode_sets_parser_error)
+{
+    struct _test {
+        StringView pattern;
+        regex::Error error;
+    };
+
+    constexpr _test tests[] {
+        { "[[]"sv, regex::Error::InvalidPattern },
+    };
+
+    for (auto test : tests) {
+        Regex<ECMA262> re(test.pattern, (ECMAScriptFlags)regex::AllFlags::UnicodeSets);
+        EXPECT_EQ(re.parser_result.error, test.error);
+    }
+}
+
 TEST_CASE(ECMA262_unicode_sets_match)
 {
     struct _test {

Userland/Libraries/LibRegex/RegexParser.cpp

@@ -2224,6 +2224,11 @@ Optional<u32> ECMA262Parser::parse_class_set_character()
         "&&"sv, "!!"sv, "##"sv, "$$"sv, "%%"sv, "**"sv, "++"sv, ",,"sv, ".."sv, "::"sv, ";;"sv, "<<"sv, "=="sv, ">>"sv, "??"sv, "@@"sv, "^^"sv, "``"sv, "~~"sv
     };
 
+    if (done()) {
+        set_error(Error::InvalidPattern);
+        return {};
+    }
+
     auto start_position = tell();
     ArmedScopeGuard restore { [&] { back(tell() - start_position + 1); } };