file: rpc_file_exec_run: fix potential memory leak and integer overflow

file: rpc_file_exec_run: fix potential memory leak and integer overflow

 - Store the realloc result in a separate pointer so that we can free
   the original on allocation failure
 - Use an explicit uint8_t for the argument vector length instead of
   "char" which might be signed or unsigned, depending on the arch
 - Bail out with an invalid argument error if the argument vector
   exceeds 255 items

Signed-off-by: Jo-Philipp Wich <jo@mein.io>