Avoid infinite recursion for unterminated non-ASCII JSON string literals.

src/json-parser.h

Index: src/json-parser.h
diff --git a/src/json-parser.h b/src/json-parser.h
index f93b04d38ffe520f0a1615b51a9efcf3c5cf21ef..68eab65fd5ab6f094cf2425ee2a95725b52c916c 100644
--- a/src/json-parser.h
+++ b/src/json-parser.h
@@ -458,12 +458,12 @@ Handle<String> JsonParser<seq_ascii>::SlowScanJsonString(
   String::WriteToFlat(*prefix, dest, start, end);
 
   while (c0_ != '"') {
+    // Check for control character (0x00-0x1f) or unterminated string (<0).
+    if (c0_ < 0x20) return Handle<String>::null();
     if (count >= length) {
       // We need to create a longer sequential string for the result.
       return SlowScanJsonString<StringType, SinkChar>(seq_str, 0, count);
     }
-    // Check for control character (0x00-0x1f) or unterminated string (<0).
-    if (c0_ < 0x20) return Handle<String>::null();
     if (c0_ != '\\') {
       // If the sink can contain UC16 characters, or source_ contains only
       // ASCII characters, there's no need to test whether we can store the