Changes

l1sap: l1sap_tch_rts_ind(): fix NULL ptr dereference

The 'resp_msg' will be NULL if msgb_dequeue_count() returns NULL,
i.e. in the case of Downlink queue underrun.  We need to handle
this gracefully and check 'resp_msg' before dereferencing it.

Change-Id: I4e1ea1d1ded2ffb3a07cc06f8b9b5dd922d32ec6
Fixes: 0a34af153 ("CSD NT modes: transmit properly aligned RLP frames on DL")