ntfs: do not dereference a null ctx on error

In ntfs_mft_data_extend_allocation_nolock(), if an error condition occurs
prior to 'ctx' being set to a non-NULL value, avoid dereferencing the NULL
'ctx' pointer in error handling.

Found by Linux Verification Center (linuxtesting.org) with SVACE.
Signed-off-by: Danila Chernetsov <listdansp@mail.ru>
Reviewed-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/ntfs/mft.c

@@ -1955,14 +1955,16 @@ static int ntfs_mft_data_extend_allocation_nolock(ntfs_volume *vol)
 				"attribute.%s", es);
 		NVolSetErrors(vol);
 	}
-	a = ctx->attr;
+
 	if (ntfs_rl_truncate_nolock(vol, &mft_ni->runlist, old_last_vcn)) {
 		ntfs_error(vol->sb, "Failed to truncate mft data attribute "
 				"runlist.%s", es);
 		NVolSetErrors(vol);
 	}
+	if (ctx) {
+		a = ctx->attr;
 		if (mp_rebuilt && !IS_ERR(ctx->mrec)) {
-		if (ntfs_mapping_pairs_build(vol, (u8*)a + le16_to_cpu(
+			if (ntfs_mapping_pairs_build(vol, (u8 *)a + le16_to_cpu(
 				a->data.non_resident.mapping_pairs_offset),
 				old_alen - le16_to_cpu(
 					a->data.non_resident.mapping_pairs_offset),
@@ -1983,8 +1985,8 @@ static int ntfs_mft_data_extend_allocation_nolock(ntfs_volume *vol)
 				"context.%s", es);
 			NVolSetErrors(vol);
 		}
-	if (ctx)
 		ntfs_attr_put_search_ctx(ctx);
+	}
 	if (!IS_ERR(mrec))
 		unmap_mft_record(mft_ni);
 	up_write(&mft_ni->runlist.lock);