linux-pipewire: Avoid trying to allocate 0 byte

[tytan652]

Description

Avoid trying to allocate 0 byte if 0 happens to be the number of format.

Motivation and Context

#11181

How Has This Been Tested?

Added a Screen Capture source, no visible issue.

Types of changes

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

• Tweak (non-breaking change to improve existing functionality)

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.