Fix prof_realloc() regression.

Mostly revert the prof_realloc() changes in
498856f4 (Move slabs out of chunks.) so
that prof_free_sampled_object() is called when appropriate.  Leave the
prof_tctx_[re]set() optimization in place, but add an assertion to
verify that all eight cases are correctly handled.  Add a comment to
make clear the code ordering, so that the regression originally fixed by
ea8d97b8 (Fix
prof_{malloc,free}_sample_object() call order in prof_realloc().) is not
repeated.

This resolves #499.