The number of local variables should be less than 1024; fix #4370

The `env` stores stack length in a 10 bit field.
See `MRB_ENV_STACK_LEN()` macro.

mrbgems/mruby-compiler/core/codegen.c

@@ -3020,6 +3020,9 @@ scope_finish(codegen_scope *s)
   mrb_state *mrb = s->mrb;
   mrb_irep *irep = s->irep;
 
+  if (s->nlocals >= 0x3ff) {
+    codegen_error(s, "too many local variables");
+  }
   irep->flags = 0;
   if (s->iseq) {
     irep->iseq = (mrb_code *)codegen_realloc(s, s->iseq, sizeof(mrb_code)*s->pc);