Fix libzip to check zip archive members pointer before freeing.

This is a patch for #1754. The actual fix is already in the master
branch of the source library:
http://hg.nih.at/libzip/comparison/8cc36648e0a2/lib/zip_fclose.c
Merge pull request #1773 from jeffchan/libzip-hotfix

Fix libzip to check zip archive members pointer before freeing