Fix edge case of extension with NULL as source string.

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index 9b33a4326c373b6508d2c097ed97e947b8a0776b..45a2ac0d8fea0a40b462f824a013b2e0489c919b 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -7386,7 +7386,7 @@ class String: public HeapObject {
 #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;
       }