Fix type of `eidx` and `ridx` from `uint8_t` to `uint16_t`; fix #4088

A byte was too small to hold ensure&rescue stacks indexes.

include/mruby.h

@@ -152,9 +152,9 @@ struct mrb_context {
   mrb_callinfo *cibase, *ciend;
 
   uint16_t *rescue;                       /* exception handler stack */
-  int rsize;
+  uint16_t rsize;
   struct RProc **ensure;                  /* ensure handler stack */
-  uint8_t esize, eidx;
+  uint16_t esize, eidx;
 
   enum mrb_fiber_state status;
   mrb_bool vmexec;