qed: don't pass NULL to memcpy

Spotted by Clang Analyzer

[Note this memcpy call has always been safe because the length will be 0
when the pointer is NULL]
Signed-off-by: NPavel Borzenkov <pavel.borzenkov@gmail.com>
Signed-off-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>

block/qed.c

@@ -1425,8 +1425,10 @@ static int bdrv_qed_change_backing_file(BlockDriverState *bs,
     memcpy(buffer, &le_header, sizeof(le_header));
     buffer_len = sizeof(le_header);
 
+    if (backing_file) {
         memcpy(buffer + buffer_len, backing_file, backing_file_len);
         buffer_len += backing_file_len;
+    }
 
     /* Write new header */
     ret = bdrv_pwrite_sync(bs->file, 0, buffer, buffer_len);