pmap: Fix minor compiler warning

pmap: Fix minor compiler warning

/usr/include/x86_64-linux-gnu/bits/error.h:40:5: warning: ‘%s’ directive argument is null [-Wformat-overflow=]
   40 |     __error_noreturn (__status, __errnum, __format, __va_arg_pack ());
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
pmap.c: In function ‘main’:
pmap.c:760:35: note: format string is defined here
  760 |         xerrx(EXIT_FAILURE, "%s: '%s'", _("failed to parse argument"), optarg);

A simple check for optarg being null silenced this, not sure why only
this one was a problem.

Signed-off-by: Craig Small <csmall@dropbear.xyz>