[3.12] gh-120155: Add assertion to sre.c match_getindex() (GH-120402) (#120410)

gh-120155: Add assertion to sre.c match_getindex() (GH-120402)

Add an assertion to help static analyzers to detect that i*2 cannot
overflow.
(cherry picked from commit 42b25dd61ff3593795c4cc2ffe876ab766098b24)

Co-authored-by: Victor Stinner <vstinner@python.org>