mgcp: Address compiler warning and initialize

mgcp/mgcp_protocol.c: In function ‘mgcp_handle_message’:
/home/ich/install/openbsc/include/osmocom/core/logging.h:59:4: warning: ‘save’ may be used uninitialized in this function [-Wmaybe-uninitialized]
    logp2(ss, level, __BASE_FILE__, __LINE__, 0, fmt, ##args); \
    ^
mgcp/mgcp_protocol.c:368:15: note: ‘save’ was declared here
  char *elem, *save;

Change-Id: I28152373b043c8a5b55a83ac09dd7bafce4d2d66
diff --git a/src/mgcp/mgcp_protocol.c b/src/mgcp/mgcp_protocol.c
index d3561d5..40acc09 100644
--- a/src/mgcp/mgcp_protocol.c
+++ b/src/mgcp/mgcp_protocol.c
@@ -365,7 +365,7 @@
 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";