Similar to r311750, check for the result from smiGetModule to avoid a segfault

when dereferencing a NULL pointer later on.

Choose to just check for the NULL pointer in the next for-loop for now to fix
the issue with a minimal amount of code churn

sys/queue.h use here would make more sense than using a static table

MFC after:	5 days

contrib/bsnmp/gensnmpdef/gensnmpdef.c

@@ -560,6 +560,8 @@ main(int argc, char *argv[])
 	level = 0;
 	last = NULL;
 	for (opt = 0; opt < argc; opt++) {
+		if (mods[opt] == NULL) /* smiGetModule failed above */
+			continue;
 		n = smiGetFirstNode(mods[opt], SMI_NODEKIND_ANY);
 		if (n == NULL)
 			continue;