gprs_sgsn.c: initialize ptmsi with 0xdeadbeef

Fix uninitialized memory access warning.
"Conditional jump or move depends on uninitialised value"
Found by valgrind.

Change-Id: Ibc2d585c5db899e6af20104211e32faf3822633a
diff --git a/openbsc/src/gprs/gprs_sgsn.c b/openbsc/src/gprs/gprs_sgsn.c
index 727524e..5d9af78 100644
--- a/openbsc/src/gprs/gprs_sgsn.c
+++ b/openbsc/src/gprs/gprs_sgsn.c
@@ -611,7 +611,7 @@
 uint32_t sgsn_alloc_ptmsi(void)
 {
 	struct sgsn_mm_ctx *mm;
-	uint32_t ptmsi;
+	uint32_t ptmsi = 0xdeadbeef;
 	int max_retries = 100;
 
 restart: