rlcmac: Fix null ptr dereference

Caught by ASan during test run. This can happen if
gprs_rlcmac_ul_tbf_schedule_next_llc_frame() finds no more llc frames to
submit during the loop iteration.

Change-Id: I5bd6dd6c6b7dc4b911af7bc119cf85eb810959a0
---
M src/rlcmac/tbf_ul.c
1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/src/rlcmac/tbf_ul.c b/src/rlcmac/tbf_ul.c
index d9cd04a..0d78383 100644
--- a/src/rlcmac/tbf_ul.c
+++ b/src/rlcmac/tbf_ul.c
@@ -619,7 +619,7 @@
 	do {
 		int payload_written = 0;
 
-		if (msgb_length(ul_tbf->llc_tx_msg) == 0) {
+		if (!ul_tbf->llc_tx_msg || msgb_length(ul_tbf->llc_tx_msg) == 0) {
 			/* The data just drained, store the current fn */
 			if (ul_tbf->last_ul_drained_fn < 0)
 				ul_tbf->last_ul_drained_fn = bi->fn;

