Fix a segfault

Found by the IBM checker.

Jeremy, you changed this from 'dirname == 0' which does not make sense at all
in the old code to '!dirname == 0' which from my point of view makes even less
sense, because the precedence of ! is according to Harbison/Steele higher than
the precedence of ==. Please check that the code now actually does what it's
supposed to do.

Thanks!

source/smbd/utmp.c

@@ -223,7 +223,7 @@ static char *uw_pathname(TALLOC_CTX *ctx,
 	}
 
 	/* For u-files and non-explicit w-dir, look for "utmp dir" */
-	if (!dirname == 0 || strlen(dirname) == 0) {
+	if ((dirname == NULL) || (strlen(dirname) == 0)) {
 		dirname = talloc_strdup(ctx, lp_utmpdir());
 		if (!dirname) {
 			return NULL;