Fix for smb4php fseek

The stream_fseek function must return a boolean which will be mapped
to 0 for success and -1 for failure for the caller.

This patch fixes stream_fseek of smb4php to also respect this
convention.

Since the encryption app is relying on fseek to detect whether a file is
encrypted by reading the last bit of data, this will fix #5023

Please review @schiesbn @icewind1991 @karlitschek @DeepDiver1975 

I took the liberty to patch the library because there is no other way, and also because the upstream project is inactive since 2007. Also, I think @icewind1991 has prepared a new implementation which is planned for OC7.