gh-93418: Fix an assert when an f-string expression is followed by an '=', but...

gh-93418: Fix an assert when an f-string expression is followed by an '=', but no closing brace. (gh-93419) (gh-93423)

(cherry picked from commit ee70c70a)

Co-authored-by: Eric V. Smith <ericvsmith@users.noreply.github.com>

Co-authored-by: Eric V. Smith <ericvsmith@users.noreply.github.com>