gitweb/gitweb.perl: don't call S_ISREG() with undef

Change S_ISREG($to_mode_oct) to S_ISREG($from_mode_oct) in the branch
that handles from modes, not to modes. This logic appears to have been
caused by copy/paste programming by Jakub Narebski in e8e41a93. It
would be better to rewrite this code not to be duplicated, but I
haven't done so.

This issue caused a failing test on perl 5.13.9, which has a warning
that turned this up:

     gitweb.perl: Use of uninitialized value in subroutine entry at /home/avar/g/git/t/../gitweb/gitweb.perl line 4415.

Which caused the Git test suite to fail on this test:

    ./t9500-gitweb-standalone-no-errors.sh             (Wstat: 256 Tests: 90 Failed: 84)
      Failed tests:  1-8, 10-36, 38-45, 47-48, 50-88
      Non-zero exit status: 1

Reported-by: perl 5.13.9
Signed-off-by: Ævar Arnfjörð Bjarmason <redacted>
Acked-by: Jakub Narebski <redacted>
Signed-off-by: Junio C Hamano <redacted>