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>

pmap.c

@@ -758,7 +758,7 @@ static void range_arguments(const char *optarg)
 
 	if ((buf = xstrdup(optarg)) == NULL) {
         xerrx(EXIT_FAILURE, "%s: '%s'", _("failed to parse argument"),
-              optarg);
+              (optarg?optarg:"(null)"));
     }
 	arg1 = buf;
 	arg2 = strchr(arg1, ',');