migration/block-dirty-bitmap: fix memory leak in dirty_bitmap_load_bits

Release buf on error path too.

Bug was introduced in b35ebdf0 "migration: add postcopy
migration of dirty bitmaps" with the whole function.
Signed-off-by: NVladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20180427142002.21930-3-vsementsov@virtuozzo.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
CC: qemu-stable@nongnu.org
Signed-off-by: NEric Blake <eblake@redhat.com>

migration/block-dirty-bitmap.c

@@ -600,6 +600,7 @@ static int dirty_bitmap_load_bits(QEMUFile *f, DirtyBitmapLoadState *s)
         ret = qemu_get_buffer(f, buf, buf_size);
         if (ret != buf_size) {
             error_report("Failed to read bitmap bits");
+            g_free(buf);
             return -EIO;
         }