media-playback: Fix null pointer dereference

[howellrl]

Description

Move channels value assignment below mp_media_can_play_frame check.

Motivation and Context

Channels value was being assigned before mp_media_can_play_frame check occurred resulting in a null pointer de-reference exception. This fixes media source crashing issue - fixes #11851.

How Has This Been Tested?

Tested by following recreation steps using attached file and observing OBS crash. Made code change and re-ran test following same recreation steps. No crash was observed after fix.

OS Version: Windows 11

Channels variable was not used in function before mp_media_can_play_frame check, moving to after the check should not affect any other areas of code.

Types of changes

Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.

[notr1ch]

This sounds oddly familiar... looks like a regression caused by what should have been a non-functional change.

I think we need to remove that assignment at the top completely if this is causing programmer confusion.

[howellrl]

Should I remove the channels assignment and pass f->ch_layout.nb_channels directly into convert_speaker_layout?

[howellrl]

I adjusted my fix to remove the channels assignment and instead pass the f->ch_layout.nb_channels directly into convert_speaker_layout.