Fix #3950: Div by 0 during DXT4 DDS load

[jessey-git]

Description

Fix the div-by-0 when handling unassociated alpha during a DXT4 DDS load. The code mirrors what other formats do by first checking to see if the alpha value is not 0 before continuing.

Tests

Added the 218-byte file from the bug to the local test suite.

Checklist:

• I have read the contribution guidelines.
• 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).
• If I added or modified a C++ API call, I have also amended the
  corresponding Python bindings (and if altering ImageBufAlgo functions, also
  exposed the new functionality as oiiotool options).
• My code follows the prevailing code style of this project. If I haven't
  already run clang-format before submitting, I definitely will look at the CI
  test that runs clang-format and fix anything that it highlights as being
  nonconforming.

[jessey-git]

There's a secondary failure after the initial one hit in the report. I'll need a bit more time to test out the fix.

The failure is that the file declares itself as a DXT4, which has alpha (4 channel), but it also has its "normal map" flag set. This confuses the code inside GetChannelCount as "normal map" takes precedence and returns 3 for the count instead of 4. This causes OIIO to allocate fewer bytes than required and ASAN rightfully flags the problem when the code eventually writes past the buffer.

[lgritz]

Is this ready to review, @jessey-git ?

[jessey-git]

Yes, I believe so.

[lgritz]

LGTM

[lgritz]

Fixes #3950