miniunz: Ensure filenames read from a ZIP file are always null terminated

[zmodem]

unzGetCurrentFileInfo64 does not null terminate the szFileName if it's longer or equal to fileNameBufferSize.

To ensure the strings are null terminated in the calling code, increase the buffer size by one and initialize with zeros.

Fixes #869

[Neustradamus]

@madler, @gvollant: Can you look this PR?

Linked to:

• https://github.com/madler/zlib/issues/869

[madler]

I fixed it by making the buffers 65537 bytes. The zip file format limits file names to 65536 bytes.

[zmodem]

I fixed it by making the buffers 65537 bytes.

Clever :-)