Fix off-by-one error introduced in r12598.

src/json-parser.h

Index: src/json-parser.h
diff --git a/src/json-parser.h b/src/json-parser.h
index ad01726415d3b47543160c521f695c2498560b0e..21c3623ef88fc5d0058c4ffdf32fcfeaa580c9cc 100644
--- a/src/json-parser.h
+++ b/src/json-parser.h
@@ -580,7 +580,7 @@ Handle<String> JsonParser<seq_ascii>::ScanJsonString() {
       if (c0_ < 0x20) return Handle<String>::null();
       running_hash = StringHasher::AddCharacterCore(running_hash, c0);
       position++;
-      if (position > source_length_) return Handle<String>::null();
+      if (position >= source_length_) return Handle<String>::null();
       c0 = seq_source_->SeqAsciiStringGet(position);
     } while (c0 != '"');
     int length = position - position_;