Fix a double free problem in codegen.c; fix #3378

This issue was first reported by https://hackerone.com/geeknik
The fix was proposed by @titanous

include/mruby/irep.h

@@ -39,6 +39,7 @@ typedef struct mrb_irep {
 
   struct mrb_locals *lv;
   /* debug info */
+  mrb_bool own_filename;
   const char *filename;
   uint16_t *lines;
   struct mrb_irep_debug_info* debug_info;