json_pointer: convert index to size_t type

json_pointer: convert index to size_t type

The index cannot be negative when parsing in is_valid_index(), because we
don't allow the '-' character in a string before we get to the strtol()
function.

So, might as well remove the negative check (for idx) in is_valid_index()
and convert it to size_t. That may allow for higher values for the index
(which can be insane, but some people may want to try it).

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>