core: Fix potentially-unused ctx var

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
diff --git a/core.c b/core.c
index 0a5a135..b0d3f05 100644
--- a/core.c
+++ b/core.c
@@ -149,7 +149,7 @@
 static struct mctp_msg_ctx *mctp_msg_ctx_create(struct mctp *mctp,
 		uint8_t src, uint8_t tag)
 {
-	struct mctp_msg_ctx *ctx;
+	struct mctp_msg_ctx *ctx = NULL;
 	unsigned int i;
 
 	for (i = 0; i < ARRAY_SIZE(mctp->msg_ctxs); i++) {