rtp: Fix memory leak fixed by Andreas without mentioning it

We allocate the msgb at entry of the method and we always need
to msgb_free it.
diff --git a/openbsc/src/libtrau/rtp_proxy.c b/openbsc/src/libtrau/rtp_proxy.c
index 82f4063..be00dde 100644
--- a/openbsc/src/libtrau/rtp_proxy.c
+++ b/openbsc/src/libtrau/rtp_proxy.c
@@ -394,7 +394,7 @@
 	rc = read(rss->bfd.fd, msg->data, RTP_ALLOC_SIZE);
 	if (rc <= 0) {
 		rss->bfd.when &= ~BSC_FD_READ;
-		return rc;
+		goto out_free;
 	}
 
 	msgb_put(msg, rc);