Need to take into account the \0 terminator of the old string to determine the amount available.

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@176948 65c4cc65-6c06-0410-ace0-fbb531ad65f3

main/utils.c

@@ -1600,7 +1600,7 @@ void __ast_string_field_ptr_build_va(struct ast_string_field_mgr *mgr,
 	*/
 	if ((*ptr)[0] != '0') {
 		target = (char *) *ptr;
-		available = strlen(target);
+		available = strlen(target) + 1;
 	} else {
 		target = (*pool_head)->base + mgr->used;
 		available = space;