Fix uninitialized pointers in alsa-mixer and rtp
alsa-mixer: Set mdev to NULL if there is no mapping
- We check if mapping is NULL but if so we never set mdev, set it to NULL as well.
Fixes: 79cb1369
rtp: Initialize adapter to NULL for early pipeline error
- If gstreamer pipeline immediately returns error, adapter pointer would not be initialized and pa_rtp_recv may crash calling gst_object_unref() on it.