pmap: minor Coverity fix for -N option

99126 Explicit null dereferenced
Not 100% sure this is valid (the same branch that sets the variable
is the one that sets N_option) but not too hard to fix.

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

pmap.c

@@ -909,10 +909,13 @@ static int config_read (char *rc_filename)
 }
 
 
-static int config_create (char *rc_filename)
+static int config_create (const char *rc_filename)
 {
 	FILE *f;
 
+	if (rc_filename == NULL)
+		return 0;
+
 	/* check if rc exists */
 	f = fopen(rc_filename, "r");