block: Fix memory leaks in bdrv_co_do_pwritev()

The error path for a failure in one of the two bdrv_aligned_preadv()
calls leaked head_buf or tail_buf, respectively. This fixes the memory
leak.
Reported-by: NLaszlo Ersek <lersek@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NLaszlo Ersek <lersek@redhat.com>

block.c

@@ -3279,9 +3279,9 @@ fail:
 
     if (use_local_qiov) {
         qemu_iovec_destroy(&local_qiov);
-        qemu_vfree(head_buf);
-        qemu_vfree(tail_buf);
     }
+    qemu_vfree(head_buf);
+    qemu_vfree(tail_buf);
 
     return ret;
 }