[bugfix] macroexpand (Issue #68)

`macroexpand()` (`src/macro.c`) restores `pic->lib` in the case of an error.
Issue #68 is fixed in this commit.

../test.scm:
```
(define-library (dummy 0)
    (import (wwq))
)

```
input and output:
```
koba-e964@debian-koba-e964-0:~/srcview/picrin$ bin/picrin 
> (import (scheme load))
=> #f
> (load "../test.scm")
error: library not found
> (+ 2 3)
=> 5
> (list 2 3)
=> (2 3)
> 
koba-e964@debian-koba-e964-0:~/srcview/picrin$ 
```