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;

mrbgems/mruby-compiler/core/codegen.c

@@ -2844,6 +2844,7 @@ scope_finish(codegen_scope *s)
     memcpy(fname, s->filename, fname_len);
     fname[fname_len] = '\0';
     irep->filename = fname;
+    irep->own_filename = TRUE;
   }
 
   irep->nlocals = s->nlocals;
@@ -2951,9 +2952,6 @@ mrb_generate_code(mrb_state *mrb, parser_state *p)
     return proc;
   }
   MRB_CATCH(&scope->jmp) {
-    if (scope->filename == scope->irep->filename) {
-      scope->irep->filename = NULL;
-    }
     mrb_irep_decref(mrb, scope->irep);
     mrb_pool_close(scope->mpool);
     return NULL;

src/state.c

@@ -159,7 +159,9 @@ mrb_irep_free(mrb_state *mrb, mrb_irep *irep)
   }
   mrb_free(mrb, irep->reps);
   mrb_free(mrb, irep->lv);
-  mrb_free(mrb, (void *)irep->filename);
+  if (irep->own_filename) {
+    mrb_free(mrb, (void *)irep->filename);
+  }
   mrb_free(mrb, irep->lines);
   mrb_debug_info_free(mrb, irep->debug_info);
   mrb_free(mrb, irep);
@@ -261,6 +263,7 @@ mrb_add_irep(mrb_state *mrb)
   irep = (mrb_irep *)mrb_malloc(mrb, sizeof(mrb_irep));
   *irep = mrb_irep_zero;
   irep->refcnt = 1;
+  irep->own_filename = FALSE;
 
   return irep;
 }