fix crash during MesonException

In commit e5a6283c4cf288fdfc9b43a92bf0ddd74dbf90f8, this function was
reorganized to assign value -> newvalue instead of overwriting newvalue,
but the error message case wasn't updated to match. As a result,
actually hitting the error would report an even more errory error, i.e.
a traceback.

(Actually hitting the error requires passing an array option as a python
object that isn't a list or a string. This is impossible to do from the
command line or meson_options.txt, but can be done with builtin options.)

0 files changed, 0 insertions, 0 deletions