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>

block/stream.c

diff --git a/block/stream.c b/block/stream.c
index 7031eef..9076203 100644
--- a/block/stream.c
+++ b/block/stream.c
@@ -155,8 +155,8 @@
 static int coroutine_fn stream_run(Job *job, Error **errp)
 {
     StreamBlockJob *s = container_of(job, StreamBlockJob, common.job);
-    BlockDriverState *unfiltered_bs;
-    int64_t len;
+    BlockDriverState *unfiltered_bs = NULL;
+    int64_t len = -1;
     int64_t offset = 0;
     int error = 0;
     int64_t n = 0; /* bytes */
@@ -177,7 +177,7 @@
 
     for ( ; offset < len; offset += n) {
         bool copy;
-        int ret;
+        int ret = -1;
 
         /* Note that even when no rate limit is applied we need to yield
          * with no pending I/O here so that bdrv_drain_all() returns.