hexdump: check blocksize when display data

hexdump: check blocksize when display data

hexdump(1) stores input to buffer and apply format unit when prints
the output. The unit can move pointer which points to the buffer, but
code does not check for limits.

Fixes: https://github.com/util-linux/util-linux/issues/2806
Signed-off-by: Karel Zak <kzak@redhat.com>
(cherry picked from commit dfa1ad272528a92384adac523cf2f2949b767d8d)