lapd: NULL pointer derefence possible here

The idiom seems to be to allocate the lapd_tei if it does not
exist and use it then. This was found by clang --analyze.
diff --git a/openbsc/src/libabis/input/lapd.c b/openbsc/src/libabis/input/lapd.c
index 2a4f60e..c8830d2 100644
--- a/openbsc/src/libabis/input/lapd.c
+++ b/openbsc/src/libabis/input/lapd.c
@@ -290,7 +290,7 @@
 		teip = teip_from_tei(li, action);
 		if (!teip) {
 			LOGP(DMI, LOGL_INFO, "TEI MGR: New TEI %u\n", action);
-			lapd_tei_alloc(li, action);
+			teip = lapd_tei_alloc(li, action);
 		}
 
 		/* Send ACCEPT */