Coverity rightly complains that checking a pointer for NULL after
dereferencing it is pointless.
Remove the check, and to be safe, assert that gst_buffer_add_meta
returns non-NULL.
CID 1455485
The message buffers are created using `gst_rtmp_message_new` and thus
always contain a GstRtmpMeta. Add checks to appease Coverity's static
analysis.
CID 1455596
CID 1455384
