Fix incorrect debug filename for bytecode from multiple *.rb

Fix #1243

With this fix bytecode from multiple *.rb with ```mrbc -g``` shows correct filename.
I hope someone to review.

```sh
[koji@macbookpro:~/work/mruby]$ cat one.rb
puts "hi from one.rb"

non_defined_func

[koji@macbookpro:~/work/mruby]$ cat two.rb
puts "hi from two.rb"

[koji@macbookpro:~/work/mruby]$ mrbc -g -o prog.mrb one.rb two.rb
[koji@macbookpro:~/work/mruby]$ mruby -b prog.mrb
hi from one.rb
trace:
	[0] one.rb:3
one.rb:3: undefined method 'non_defined_func' for main (NoMethodError)
```