Fix double-free issue in File::create

Triggered when opening a file from the command line.

During startup Sigsession::set_default_device calls Device::use which
loads a global _sr_session, then as part of file loading the
File::create method is called which treats _sr_session as a temp
variable (loaded then immediately released), finally a Device::release
releases the (differently allocated) global _sr_session again causing
the double free.

Given File::create is only using the sigrok session temporarily within
its function scope, this change gives it its own temporary session
instance.

This fixes bug #405.
Merge remote-tracking branch 'upstream/master'