bsc_ctrl: Ensure we don't pass NULL string into strtok_r()

Change-Id: I03bea132377c0136b55b6fdad99a5d92da12e692
Fixes: Coverity CID 135180
diff --git a/openbsc/src/osmo-bsc/osmo_bsc_ctrl.c b/openbsc/src/osmo-bsc/osmo_bsc_ctrl.c
index f10a74a..5352bf2 100644
--- a/openbsc/src/osmo-bsc/osmo_bsc_ctrl.c
+++ b/openbsc/src/osmo-bsc/osmo_bsc_ctrl.c
@@ -605,6 +605,7 @@
 	char *saveptr = NULL;
 	char *inp, *cic, *alert, *text;
 
+	OSMO_ASSERT(cmd);
 	inp = talloc_strdup(cmd, value);
 
 	cic = strtok_r(inp, ",", &saveptr);