qcacmn: Add sanity check to avoid NULL pointer dereference

Propagation from qcacld-2.0 to qcacmn.

Add sanity check to avoid NULL pointer dereferencing of tx
completion handler callback in hif_send_buffer_cleanup_on_pipe.

Change-Id: If38b4b7cdd1154d8272e5150c6653f90df8f4b5e
CRs-Fixed: 1057490

hif/src/ce/ce_main.c

@@ -1808,8 +1808,10 @@ void hif_send_buffer_cleanup_on_pipe(struct HIF_CE_pipe_info *pipe_info)
 				return;
 			/* Indicate the completion to higher
 			 * layer to free the buffer */
-			hif_state->msg_callbacks_current.
-			txCompletionHandler(hif_state->
+			if (hif_state->msg_callbacks_current.
+					txCompletionHandler)
+				hif_state->msg_callbacks_current.
+				    txCompletionHandler(hif_state->
 					    msg_callbacks_current.Context,
 					    netbuf, id, toeplitz_hash_result);
 		}