Issue #12121: Only restrict palette for indexed images

This patch attempts to resolve issue #12121 (closed).

gimp_palette_restrict_format () fails if the palette is NULL, which is the case for non-indexed images.

This patch just moves the call until after we've verified that private->palette is not NULL. This is a very simple fix, but since @Jehan just wrote this code, I'm leaving it for testing in case I'm missing something about why the code is currently structured this way.