FLIP: Fix use-after-free and double frees with temporary transformations.

I originally thought this was a bug in FLIP, but it turned out that in
fact we just hadn't ever really tested
proc_make_temporary_transformations_permanent() properly with actual
transformations, and especially with adding a transformation after calling
proc_make_temporary_transformations_permanent().

The modified test should avoid regression for the bug.

Bug #38832.