Make sure the fast case of ScanJsonString bails out to the slow case correctly.

src/json-parser.h

 // Copyright 2011 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 562 matching lines @@
 
   if (seq_ascii && is_symbol) {
     // Fast path for existing symbols.  If the the string being parsed is not
     // a known symbol, contains backslashes or unexpectedly reaches the end of
     // string, return with an empty handle.
     uint32_t running_hash = isolate()->heap()->HashSeed();
     int position = position_;
     uc32 c0 = c0_;
     do {
       if (c0 == '\\') {
+        c0_ = c0;
+        int beg_pos = position_;
+        position_ = position;
         return SlowScanJsonString<SeqAsciiString, char>(source_,
-                                                        position_,
-                                                        position);
+                                                        beg_pos,
+                                                        position_);
       }
       if (c0 < 0x20) return Handle<String>::null();
       running_hash = StringHasher::AddCharacterCore(running_hash, c0);
       position++;
       if (position >= source_length_) return Handle<String>::null();
       c0 = seq_source_->SeqAsciiStringGet(position);
     } while (c0 != '"');
     int length = position - position_;
     uint32_t hash = (length <= String::kMaxHashCalcLength)
         ? StringHasher::GetHashCore(running_hash) : length;
@@ skipping 53 matching lines @@
   }
   ASSERT_EQ('"', c0_);
   // Advance past the last '"'.
   AdvanceSkipWhitespace();
   return result;
 }
 
 } }  // namespace v8::internal
 
 #endif  // V8_JSON_PARSER_H_