Summary and Info:
alpm crashes on passing NULL to alpm_trans_commit in a sync operation.

I accidentally passed NULL to alpm_trans_commit in a sync operation (very accidentally by writing NULL), and alpm segfaulted. Of course, it's nothing expected.

Since a compiler won't warn about NULL passed to this function (it's expected on add), i think alpm should be able to handle this properly and return an error.

Steps to Reproduce:
- Initialize a sync transaction
- commit it using NULL as parameter