res_pjsip: Fixed invalid empty Server and User-Agent SIP headers.

Setting pjsip.conf useragent to an empty string results in an empty SIP
header being sent.

* Made not add an empty SIP header item to the global SIP headers list.

Review: https://reviewboard.asterisk.org/r/4467/
........

Merged revisions 432764 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@432765 65c4cc65-6c06-0410-ace0-fbb531ad65f3

res/res_pjsip/pjsip_global_headers.c

@@ -121,18 +121,22 @@ static void remove_header(struct header_list *headers, const char *to_remove)
 
 static int add_header(struct header_list *headers, const char *name, const char *value, int replace)
 {
-	struct header *to_add;
+	struct header *to_add = NULL;
 
-	to_add = alloc_header(name, value);
-	if (!to_add) {
-		return -1;
+	if (!ast_strlen_zero(value)) {
+		to_add = alloc_header(name, value);
+		if (!to_add) {
+			return -1;
+		}
 	}
 
 	AST_RWLIST_WRLOCK(headers);
 	if (replace) { 
 		remove_header(headers, name);
 	}
-	AST_LIST_INSERT_TAIL(headers, to_add, next);
+	if (to_add) {
+		AST_LIST_INSERT_TAIL(headers, to_add, next);
+	}
 	AST_RWLIST_UNLOCK(headers);
 
 	return 0;