Stack position may be bigger than stack bottom; fix #3401

This issue was reported by https://hackerone.com/titanous

1 files changed, 1 insertions, 0 deletions

diff --git a/src/vm.c b/src/vm.c
index c32fb0c0a..77372d937 100644
--- a/src/vm.c
+++ b/src/vm.c
@@ -137,6 +137,7 @@ stack_extend_alloc(mrb_state *mrb, int room, int keep)
   size_t size = mrb->c->stend - mrb->c->stbase;
   size_t off = mrb->c->stack - mrb->c->stbase;
 
+  if (off > size) size = off;
 #ifdef MRB_STACK_EXTEND_DOUBLING
   if (room <= size)
     size *= 2;