readelf: Fix bounds check in print_form_data.

readelf: Fix bounds check in print_form_data.

The afl fuzzer found that we did a wrong check in print_form_data when
comparing the remaining bytes in the buffer to an (unsigned) value read.
We were casting the value to ptrdiff_t which is a signed value and so
might turn a really big unsigned value into a negative number. Since we
know the difference between readendp and readp is zero or greater, we
should cast the pointer difference to size_t (and unsigned type) instead
before comparing with the unsigned value.

Signed-off-by: Mark Wielaard <mark@klomp.org>