block/stream: fix -Werror=maybe-uninitialized false-positives

block/stream: fix -Werror=maybe-uninitialized false-positives

../block/stream.c:193:19: error: ‘unfiltered_bs’ may be used uninitialized [-Werror=maybe-uninitialized]
../block/stream.c:176:5: error: ‘len’ may be used uninitialized [-Werror=maybe-uninitialized]
trace/trace-block.h:906:9: error: ‘ret’ may be used uninitialized [-Werror=maybe-uninitialized]

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Acked-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Reviewed-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>