bpo-46070: Fix asyncio initialisation guard (GH-30423)

If init flag is set, exit successfully immediately.
If not, only set the flag after successful initialization.
(cherry picked from commit b127e70a8a682fe869c22ce04c379bd85a00db67)

Co-authored-by: Erlend Egeberg Aasland <erlend.aasland@innova.no>