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 "
diff --git a/src/tbf.cpp b/src/tbf.cpp
index cac807c..1568720 100644
--- a/src/tbf.cpp
+++ b/src/tbf.cpp
@@ -568,6 +568,7 @@
 				LOGP(DRLCMAC, LOGL_NOTICE, "%s releasing due to "
 					"PACCH assignment timeout.\n", tbf_name(this));
 				tbf_free(this);
+				return;
 			} else
 				LOGP(DRLCMAC, LOGL_ERROR, "Error: %s is not "
 					"in assign state\n", tbf_name(this));
@@ -595,6 +596,7 @@
 			"%s will be freed due to timeout\n", tbf_name(this));
 		/* free TBF */
 		tbf_free(this);
+		return;
 		break;
 	default:
 		LOGP(DRLCMAC, LOGL_ERROR,
@@ -1531,6 +1533,7 @@
 			"Killing pending DL TBF\n", tlli(),
 			tbf_name(dl_tbf));
 		tbf_free(dl_tbf);
+		dl_tbf = NULL;
 	}
 	/* tbf_by_tlli will not find your TLLI, because it is not
 	 * yet marked valid */
@@ -1540,6 +1543,7 @@
 			"Killing pending UL TBF\n", tlli(),
 			tbf_name(ul_tbf));
 		tbf_free(ul_tbf);
+		ul_tbf = NULL;
 	}
 	/* mark TLLI valid now */
 	tlli_mark_valid();