[3.13] gh-145743: Fix inconsistency after calling Struct.__init__() with invalid format (GH-145744) (GH-145764)

Only set the format attribute after successful (re-)initialization.
(cherry picked from commit 3f33bf83e8496737b86333bc9ec55dc3ccb3faca)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>