fix #1550

emit a "\n" as the first token for parser instead of taking the
first character from the next file for lexer, to prevent mruby
"concatenate" two keywords between files

it should be regarded as a work around, for it breaks abstraction
layers, and can not resolve this case:

```
$ cat a.rb
"
$ cat b.rb
b"

$ bin/mrbc -o- -v a.rb b.rb
mruby - Embeddable Ruby  Copyright (c) 2010-2013 mruby
developers
NODE_SCOPE:
        NODE_BEGIN:
                NODE_STR "

b " len 4
irep 0 nregs=2 nlocals=1 pools=1 syms=0
000 OP_STRING   R1      "\n\nb "
001 OP_STOP
```


thanks @bovi 's idea