gofrontend: correct file reading logic in Stream_from_file
gofrontend: correct file reading logic in Stream_from_file

The implementation of Stream_from_file mishandled several cases:

  * It reversed the check for whether bytes were already available in
    the peek buffer.

  * It considered positive return values from lseek to be an error, when
    only a -1 return value indicates an error.

Change-Id: Ic699d3ff7fe589f5ca2362b3db2971a6b9dc9022
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/259437
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Trust: Cherry Zhang <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>

golang/gofrontend,golang/gofrontend,golang/gofrontend,golang/gofrontend