manual: Fix backtraces code example [BZ #10441]

manual: Fix backtraces code example [BZ #10441]

Validation for pointer returned by backtrace_symbols () added.
Type of variables size and i is changed from size_t to int.

Variable size is used to collect the result from backtrace ()
that is an int. i is the loop counter variable so it can be an int.

Since, size_t size is changed to int size, in printf %zd is changed to %d.

Reviewed-by: DJ Delorie <dj@redhat.com>