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

src/json-parser.h

Index: src/json-parser.h
diff --git a/src/json-parser.h b/src/json-parser.h
index d940c140464bd2c1fe567eeafc75231f39ef2514..80991f0c236cad4886a039be9eccd055040386f5 100644
--- a/src/json-parser.h
+++ b/src/json-parser.h
@@ -580,9 +580,12 @@ Handle<String> JsonParser<seq_ascii>::ScanJsonString() {
     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);