fs_mgr: check that fstab is not NULL in fs_mgr_free_fstab

Make sure fstab is not NULL before freeing it

Bug: 10911605
Change-Id: I549c0a470dd183fb15a2f3c5cf4f3dd393b6e307

fs_mgr/fs_mgr.c

@@ -430,6 +430,10 @@ void fs_mgr_free_fstab(struct fstab *fstab)
 {
     int i;
 
+    if (!fstab) {
+        return;
+    }
+
     for (i = 0; i < fstab->num_entries; i++) {
         /* Free the pointers return by strdup(3) */
         free(fstab->recs[i].blk_device);