subscr_name(): Handle case for subscr == NULL

subscr_name() was called from several places:
* either without a check for subscr being NULL, which for example
  was causing a segfault if we hand-over a channel before identifying the
  subscriber
* or with an explicit NULL check and the ternary operator (?).

We now simplify the code by checking for the NULL Subscriber in subscr_name()
itself.

Change-Id: Ide09f4a515222eb2ec6c25e7a6a8c5f6cc2ffd4b
Reviewed-on: https://gerrit.osmocom.org/92
Tested-by: Jenkins Builder
Reviewed-by: Holger Freyther <holger@freyther.de>
diff --git a/openbsc/src/libcommon/gsm_subscriber_base.c b/openbsc/src/libcommon/gsm_subscriber_base.c
index 50f4118..1f98cc6 100644
--- a/openbsc/src/libcommon/gsm_subscriber_base.c
+++ b/openbsc/src/libcommon/gsm_subscriber_base.c
@@ -43,6 +43,9 @@
 
 char *subscr_name(struct gsm_subscriber *subscr)
 {
+	if (!subscr)
+		return "unknown";
+
 	if (strlen(subscr->name))
 		return subscr->name;