[CHANGESET] Random failure of fopen when file does not exist

[Rafael Laboissiere]

In the process of packaging octave 3.1.51 for Debian, I am getting errors
like this:

/usr/bin/make -C interpreter all
[...]
../../run-octave -f -q -H -p . --eval "sparseimages ('spchol', 'txt');"
error: fputs: invalid stream number = -1
[...]

The annoying thing is that the bug is not deterministic and the build fails
each time on a different run-octave line.

I think I nailed down the problem and the changeset attached below should
fix it.  It seems to be caused by changeset b68e44c90afe, in which a test
using fs.is_dir () was introduced in do_stream_open().  The problem is that
when the file_stat fs variable is created in file_stat fs (fname) and the
file with name fname does not exist (as it is the case in the example
above), then the private variable fs_mode is not initialized.  In my system
here (Debian sid amd64) it seems that fs.is_dir() will return random values
in this case.

I hope I did the correct analysis and the fix is appropriate.  At least, the
Debian package is building correctly now.

-- 
Rafael
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