Fix off-by-1 read err in switch_xml

git-svn-id: http://svn.freeswitch.org/svn/freeswitch/trunk@12628 d0543943-73ff-0310-b7d9-9358b9ac24b2

src/switch_xml.c

@@ -2035,7 +2035,7 @@ static char *switch_xml_toxml_r(switch_xml_t xml, char **s, switch_size_t *len,
 		*s = tmp;
 	}
 
-	if (*(*s + (*len) - 1) == '>') {
+	if (*len && *(*s + (*len) - 1) == '>') {
 		*len += sprintf(*s + *len, "\n");	/* indent */
 	}
 	for (lcount = 0; lcount < *count; lcount++) {