Fix minor gcc warning when -O3 is enabled

Author: mgreter

src/json.cpp

@@ -802,7 +802,7 @@ static bool parse_object(const char **sp, JsonNode **out)
 bool parse_string(const char **sp, char **out)
 {
   const char *s = *sp;
-  SB sb;
+  SB sb = { 0, 0, 0};
   char throwaway_buffer[4];
     /* enough space for a UTF-8 character */
   char *b;