Avoid harmless warning about comparing unsigned with 0.

Fix warning in assert in wxProcess::SetPriority(): don't compare unsigned
priority with wxPRIORITY_MIN which is just 0, the condition is always true.

Closes #14931.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73456 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775