should adjust stack size when irep->nregs is larger than allocate size;

close #835

src/vm.c

@@ -108,6 +108,9 @@ stack_extend(mrb_state *mrb, int room, int keep)
     size = mrb->stend - mrb->stbase;
     off = mrb->stack - mrb->stbase;
 
+    /* do not leave uninitialized malloc region */
+    if (keep > size) keep = size;
+
     /* Use linear stack growth.
        It is slightly slower than doubling thestack space,
        but it saves memory on small devices. */
@@ -554,6 +557,7 @@ mrb_run(mrb_state *mrb, struct RProc *proc, mrb_value self)
   if (!mrb->stack) {
     stack_init(mrb);
   }
+  stack_extend(mrb, irep->nregs, irep->nregs);
   mrb->ci->proc = proc;
   mrb->ci->nregs = irep->nregs + 2;
   regs = mrb->stack;