Use scoped_value_mark in two more places

Use scoped_value_mark in two more places

I found a couple of spots that could use scoped_value_mark.  One of
them is a spot that didn't consider the possibility that value_mark
can return NULL.  I tend to doubt this can be seen in this context,
but nevertheless this is safer.

Regression tested on x86-64 Fedora 36.