qcow2: fix null pointer dereference

It seems this assert() was somehow misplaced.

block/qcow2-refcount.c:2193:42: warning: Array access (from variable 'on_disk_reftable') results in a null pointer dereference
        on_disk_reftable[refblock_index] = refblock_offset;
        ~~~~~~~~~~~~~~~~                 ^

Reported-by: Clang Static Analyzer
Signed-off-by: Philippe Mathieu-Daudé <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Signed-off-by: Michael Tokarev <[email protected]>

block/qcow2-refcount.c

@@ -2189,6 +2189,8 @@ static int rebuild_refcount_structure(BlockDriverState *bs,
              * this will leak that range, but we can easily fix that by running
              * a leak-fixing check after this rebuild operation */
             reftable_offset = -1;
+        } else {
+            assert(on_disk_reftable);
         }
         on_disk_reftable[refblock_index] = refblock_offset;
 
@@ -2258,8 +2260,6 @@ static int rebuild_refcount_structure(BlockDriverState *bs,
         goto write_refblocks;
     }
 
-    assert(on_disk_reftable);
-
     for (refblock_index = 0; refblock_index < reftable_size; refblock_index++) {
         cpu_to_be64s(&on_disk_reftable[refblock_index]);
     }