mgcp: Make sure the save pointer is initialized with NULL.

Coverity is not happy about it but it doesn't appear to be a real
issue as the data will not be NULL on the first call.

Addresses: Coverity CID 1040704

openbsc/src/libmgcp/mgcp_protocol.c

@@ -337,7 +337,7 @@ static struct mgcp_endpoint *find_endpoint(struct mgcp_config *cfg, const char *
 static int mgcp_analyze_header(struct mgcp_parse_data *pdata, char *data)
 {
 	int i = 0;
-	char *elem, *save;
+	char *elem, *save = NULL;
 
 	pdata->trans = "000000";