sgsn: check for NULL of gprs_subscr_get_or_create()

gprs_subscr_get_or_create() can return NULL if no memory can
be allocated. Detected by the compiler on Ubuntu s390x.

Signed-off-by: Steve Langasek <steve.langasek@ubuntu.com>
Signed-off-by: Alexander Couzens <lynxis@fe80.eu>

Change-Id: I86b3652d46bdd581fe6cbab16b52395a0daaa082
diff --git a/src/sgsn/sgsn_vty.c b/src/sgsn/sgsn_vty.c
index 33a652c..fee328e 100644
--- a/src/sgsn/sgsn_vty.c
+++ b/src/sgsn/sgsn_vty.c
@@ -1020,6 +1020,10 @@
 	}
 
 	subscr = gprs_subscr_get_or_create(imsi);
+	if (!subscr) {
+		vty_out(vty, "Can not create subscriber. Out of memory.%s", imsi);
+		return CMD_WARNING;
+	}
 	subscr->keep_in_ram = 1;
 	gprs_subscr_put(subscr);