Fix edge case of extension with NULL as source string.

src/objects.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 7368 matching lines @@
   static void WriteToFlat(String* source,
                           sinkchar* sink,
                           int from,
                           int to);
 
   static inline bool IsAscii(const char* chars, int length) {
     const char* limit = chars + length;
 #ifdef V8_HOST_CAN_READ_UNALIGNED
     ASSERT(kMaxAsciiCharCode == 0x7F);
     const uintptr_t non_ascii_mask = kUintptrAllBitsSet / 0xFF * 0x80;
-    while (chars <= limit - sizeof(uintptr_t)) {
+    while (chars + sizeof(uintptr_t) <= limit) {
       if (*reinterpret_cast<const uintptr_t*>(chars) & non_ascii_mask) {
         return false;
       }
       chars += sizeof(uintptr_t);
     }
 #endif
     while (chars < limit) {
       if (static_cast<uint8_t>(*chars) > kMaxAsciiCharCodeU) return false;
       ++chars;
     }
@@ skipping 1552 matching lines @@
     } else {
       value &= ~(1 << bit_position);
     }
     return value;
   }
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_OBJECTS_H_