jail(8): stop crashing with SIGSEGV inside run_command() function

while processing not entirely correct jail.conf(5) file
having something like "ip4.addr = 127.0.0.1;" and no "ip4 = ...;"
so extrap variable stays NULL.

Reported by:	marck
MFC after:	1 month

usr.sbin/jail/command.c

@@ -374,7 +374,7 @@ run_command(struct cfjail *j)
 			argc = 4;
 		}
 
-		if (!down) {
+		if (!down && extrap != NULL) {
 			for (cs = strtok(extrap, " "); cs;
 			     cs = strtok(NULL, " ")) {
 				size_t len = strlen(cs) + 1;