pcu_sock: fix memleak

pcu_sock_read() may not free the message buffer in case the recv
returned errno EAGAIN. This is already fixed in osmo-bts, lets fix it in
osmo-bsc as well.

Related: OS#5198
Change-Id: I49eda447fc1912c1f7f25ba07331cb84decf4548

src/osmo-bsc/pcu_sock.c

@@ -567,8 +567,10 @@ static int pcu_sock_read(struct osmo_fd *bfd)
 		goto close;
 
 	if (rc < 0) {
-		if (errno == EAGAIN)
+		if (errno == EAGAIN) {
+			msgb_free(msg);
 			return 0;
+		}
 		goto close;
 	}