block: fix theoretical overflow in bdrv_init_padding()

Calculation of sum may theoretically overflow, so use 64bit type and
add some good assertions.

Use int64_t constantly.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20201211183934.169161-4-vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
[eblake: tweak assertion order]
Signed-off-by: Eric Blake <eblake@redhat.com>

block/io.c

@@ -1565,8 +1565,12 @@ static bool bdrv_init_padding(BlockDriverState *bs,
                               int64_t offset, int64_t bytes,
                               BdrvRequestPadding *pad)
 {
-    uint64_t align = bs->bl.request_alignment;
-    size_t sum;
+    int64_t align = bs->bl.request_alignment;
+    int64_t sum;
+
+    bdrv_check_request(offset, bytes, &error_abort);
+    assert(align <= INT_MAX); /* documented in block/block_int.h */
+    assert(align <= SIZE_MAX / 2); /* so we can allocate the buffer */
 
     memset(pad, 0, sizeof(*pad));