Index: main/strings.c
===================================================================
--- main/strings.c	(revision 185947)
+++ main/strings.c	(working copy)
@@ -143,7 +143,8 @@
 		maxlen--;
 		(*buf)->__AST_STR_USED++;
 
-		if (dynamic && (!maxlen || (escapecommas && !(maxlen - 1)))) {
+		if ((ptr >= (*buf)->__AST_STR_STR + (*buf)->__AST_STR_LEN - 3) ||
+			(dynamic && (!maxlen || (escapecommas && !(maxlen - 1))))) {
 			char *oldbase = (*buf)->__AST_STR_STR;
 			size_t old = (*buf)->__AST_STR_LEN;
 			if (ast_str_make_space(buf, (*buf)->__AST_STR_LEN * 2)) {