Failing to open in-memory files

[murinrad]

Seems that projects which are using this library cannot open file descriptors which point to an in memory file (named pipes).

After some investigation, it seems that there is a check in the nativeOpenDocument method which verifies the file descriptor by checking the file size. If it it zero, then the method throws an exception.

However a zero file length is valid for in memory files and their file descriptors.
It would be nice if these descriptors were supported.

https://github.com/barteksc/PdfiumAndroid/blob/master/src/main/jni/src/mainJNILib.cpp#L183