Fix integer overflow check in json code

* src/json.c (json_to_lisp): Check for ptrdiff_t overflow,
not fixnum overflow.
1 job for master in 22 minutes and 28 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
failed #428
test

00:22:28

 
Name Stage Failure
failed
test Test
make[2]: *** [check-doit] Error 1
Makefile:309: recipe for target 'check-doit' failed
make[2]: Leaving directory '/builds/emacs/emacs/test'
Makefile:279: recipe for target 'check' failed
make[1]: *** [check] Error 2
make[1]: Leaving directory '/builds/emacs/emacs/test'
Makefile:955: recipe for target 'check' failed
make: *** [check] Error 2
ERROR: Job failed: exit code 1