tbf: Make finding use-after-free more easy and set to NULL or return

Make finding use-after-free more easy by setting things to NULL
or simply return after tbf_free(this) has been called.
diff --git a/src/bts.cpp b/src/bts.cpp
index 7c60452..ff16e29 100644
--- a/src/bts.cpp
+++ b/src/bts.cpp
@@ -851,6 +851,7 @@
 				"Killing pending DL TBF\n", tlli,
 				tbf_name(dl_tbf));
 			tbf_free(dl_tbf);
+			dl_tbf = NULL;
 		}
 		LOGP(DRLCMAC, LOGL_DEBUG, "MS requests UL TBF "
 			"in packet resource request of single "