[3.13] gh-128636: Fix crash in PyREPL when os.environ is overwritten with an invalid value for macOS (GH-138089)

(cherry picked from commit 8ef7735)

Co-authored-by: yihong <[email protected]>
Signed-off-by: yihong0618 <[email protected]>
Co-authored-by: Bénédikt Tran <[email protected]>

Author: 3 people