hexdump: fix potential null pointer dereference warnings

hexdump: fix potential null pointer dereference warnings

First three fixes on lines 133, 151, and 280 are cosmetic.  Because there
was unobvious null check compiler thought variable might be null, and warned
when after pointer adjustment it was followed without null check.  Perhaps
this will not happen sometime in future when compiler is made more smart,
meanwhile lets give better hints to avoid false positive.

The last change addresses issue that is possible, at least in theory.

text-utils/hexdump-parse.c:465:12: warning: potential null pointer
dereference [-Wnull-dereference]

Signed-off-by: Sami Kerola <kerolasa@iki.fi>