Fix error checking for file descriptor in gifinput on windows

[SRHMorris]

Description

Calling ImageInput::open() with a file path that doesn't exist will cause OIIO to attempt to load the missing file with all plugins. When it gets to the gif plugin the file descriptor is not checked correctly (windows only). This causes an assert inside DGifOpenFileHandle() when it calls _setmode().

Tests

Checklist:

• I have read the contribution guidelines.
• If this is more extensive than a small change to existing code, I
  have previously submitted a Contributor License Agreement
  (individual, and if there is any way my
  employers might think my programming belongs to them, then also
  corporate).
• I have updated the documentation, if applicable.
• I have ensured that the change is tested somewhere in the testsuite
  (adding new test cases if necessary).
• My code follows the prevailing code style of this project.

[lgritz]

Good catch!