LibGUI: Don't silently create a 0 UIDimension when the JSON is invalid

The function already can report an invalid JSON value for the dimension,
so let's actually use that for when the number is too large or some
other invalid JSON type, like an object or a boolean, was passed.

Userland/Libraries/LibGUI/UIDimensions.h

@@ -162,12 +162,13 @@ public:
                 return UIDimension { SpecialDimension::Fit };
             else
                 return {};
-        } else {
-            int value_int = value.to_i32();
+        } else if (value.is_integer<i32>()) {
+            auto value_int = value.as_integer<i32>();
             if (value_int < 0)
                 return {};
             return UIDimension(value_int);
         }
+        return {};
     }
 
 private: