block: Fix compiler warning (-Werror=uninitialized)

The patch fixes a warning from gcc (Debian 4.6.3-14+rpi1) 4.6.3:

block/stream.c:141:22: error:
‘copy’ may be used uninitialized in this function [-Werror=uninitialized]

This is not a real bug - a better compiler would not complain.

Now 'copy' has always a defined value, so the check for ret >= 0
can be removed.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>