luks: Turn another invalid assertion into check

Commit e39e959e fixed an invalid assertion in the .bdrv_length
implementation, but left a similar assertion in place for
.bdrv_truncate. Instead of crashing when the user requests a too large
image size, fail gracefully.

A file size of exactly INT64_MAX caused failure before, but is actually
legal.
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>

block/crypto.c

@@ -357,7 +357,11 @@ static int block_crypto_truncate(BlockDriverState *bs, int64_t offset,
     BlockCrypto *crypto = bs->opaque;
     uint64_t payload_offset =
         qcrypto_block_get_payload_offset(crypto->block);
-    assert(payload_offset < (INT64_MAX - offset));
+
+    if (payload_offset > INT64_MAX - offset) {
+        error_setg(errp, "The requested file size is too large");
+        return -EFBIG;
+    }
 
     offset += payload_offset;