btrfs: avoid NULL deref in btrfs_reserve_extent with DEBUG_ENOSPC

 __find_space_info can return NULL but we don't check it before calling
 dump_space_info().

Signed-off-by: Jeff Mahoney <jeffm@suse.com>

fs/btrfs/extent-tree.c

@@ -5838,7 +5838,8 @@ again:
 			printk(KERN_ERR "btrfs allocation failed flags %llu, "
 			       "wanted %llu\n", (unsigned long long)data,
 			       (unsigned long long)num_bytes);
-			dump_space_info(sinfo, num_bytes, 1);
+			if (sinfo)
+				dump_space_info(sinfo, num_bytes, 1);
 		}
 	}