clear local variables for the first execution of mrb_context_run(); close #2405

include/mruby/compile.h

@@ -28,6 +28,7 @@ typedef struct mrbc_context {
   mrb_bool capture_errors:1;
   mrb_bool dump_result:1;
   mrb_bool no_exec:1;
+  mrb_bool keep_lv:1;
 } mrbc_context;
 
 mrbc_context* mrbc_context_new(mrb_state *mrb);

src/parse.y

@@ -5558,7 +5558,12 @@ load_exec(mrb_state *mrb, parser_state *p, mrbc_context *c)
     if (c->target_class) {
       target = c->target_class;
     }
-    keep = c->slen + 1;
+    if (c->keep_lv) {
+      keep = c->slen + 1;
+    }
+    else {
+      c->keep_lv = TRUE;
+    }
   }
   proc->target_class = target;
   if (mrb->c->ci) {