[3.14] gh-142218: Fix split table dictionary crash (gh-142229) (gh-142244)

This fixes a regression introduced in gh-140558. The interpreter would
crash if we inserted a non `str` key into a split table that matches an
existing key.
(cherry picked from commit 547d8daf780646e2800bec598ed32085817c8606)

Co-authored-by: Sam Gross <colesbury@gmail.com>