Avoid warnings about uninitialized variables in TGA loading code.

Avoid warnings about uninitialized variables in TGA loading code.

Initialize the variables containing the colour components: even though they
should normally be always filled by Palette_GetRGB() call below, this
presumably might not happen for a corrupted image with invalid palette table
entries and g++ correctly complains about it.

See #14459.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71975 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775