[media] videobuf2-core: Replace BUG_ON and return an error at vb2_queue_init()

This replaces BUG_ON() calls with WARN_ON(), and returns
EINVAL if some parameter is NULL, as suggested by Jonathan and Mauro.
The BUG_ON() call is too drastic to be used in this case.
See the full discussion here:
http://www.spinics.net/lists/linux-media/msg52462.html

Signed-off-by: Ezequiel Garcia <elezegarcia@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>
diff --git a/include/media/videobuf2-core.h b/include/media/videobuf2-core.h
index 8dd9b6c..e04252a 100644
--- a/include/media/videobuf2-core.h
+++ b/include/media/videobuf2-core.h
@@ -324,7 +324,7 @@
 int vb2_create_bufs(struct vb2_queue *q, struct v4l2_create_buffers *create);
 int vb2_prepare_buf(struct vb2_queue *q, struct v4l2_buffer *b);
 
-int vb2_queue_init(struct vb2_queue *q);
+int __must_check vb2_queue_init(struct vb2_queue *q);
 
 void vb2_queue_release(struct vb2_queue *q);