Fix use-after-free

Fix use-after-free

Building Tvheadend on Raspberry Pi with gcc 12 fails with this error

src/misc/json.c: In function ‘json_parse_string’:
src/misc/json.c:120:31: error: pointer ‘r’ used after ‘free’ [-Werror=use-after-free]
  120 |                   *failp = (a - r) + start;
      |                            ~~~^~~~
src/misc/json.c:118:19: note: call to ‘free’ here
  118 |                   free(r);
      |                   ^~~~~~~

This PR appears correct and fixes the gcc error but has not been tested as it is an error path.