Prevent Crash in show pdp-context from vty

Fix test for return value from gtp_pdp_getimsi() so
we do not call show_one_pdp() with an uninitialised pdp_t

Change-Id: Ic40429939b185f97c020dd3904e054fe860b91e8
diff --git a/ggsn/ggsn_vty.c b/ggsn/ggsn_vty.c
index 91ffe4e..fd2c6ad 100644
--- a/ggsn/ggsn_vty.c
+++ b/ggsn/ggsn_vty.c
@@ -903,7 +903,7 @@
 
 	if (argc > 2) {
 		nsapi = atoi(argv[2]);
-		if (gtp_pdp_getimsi(ggsn->gsn, &pdp, imsi, nsapi)) {
+		if (!gtp_pdp_getimsi(ggsn->gsn, &pdp, imsi, nsapi)) {
 			show_one_pdp(vty, pdp);
 			num_found++;
 		}