mount: do not execute "mount.(null)" :)

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/util-linux/mount.c b/util-linux/mount.c
index 56c32e1..478dc24 100644
--- a/util-linux/mount.c
+++ b/util-linux/mount.c
@@ -448,7 +448,7 @@
 
 		// If mount failed, try
 		// helper program mount.<mnt_type>
-		if (HELPERS_ALLOWED && rc) {
+		if (HELPERS_ALLOWED && rc && mp->mnt_type) {
 			char *args[8];
 			int errno_save = errno;
 			args[0] = xasprintf("mount.%s", mp->mnt_type);