Fix intermittent segfault when reading zip files

When looking up the compression name for a zip file the index variable was being
incremented too soon. Thus element zero ("uncompressed") was never checked and
reads could be made past the end of the array. This was causing intermittent
segfaults in the call to sprintf in zip_read_local_file_header.

Signed-off-by: Paul Barker <paul@paulbarker.me.uk>
Use snprintf to print to zip->format_name

zip->format_name is a fixed size character array so ensure that the number of
characters written to the array is appropriately limited.

Signed-off-by: Paul Barker <paul@paulbarker.me.uk>
Merge pull request #72 from betafive/pbarker/fix-zip

Fix intermittent segfault when reading zip files