Fix infinite loop in psgi/coroae

Perl PSGI app infinite loops since f20213ff10d1980a97c5c72c2e56044a05f21f8c

Looking at that change, other loop termination conditions are '<= UWSGI_OK'.
Using gdb to inspect the app while looping shows thats ->request is returning
UWSGI_OK in this case. However, the numeric value of this is zero, so we
never exit the for(;;) loop.

Cargo culting loop escape conditions from core/async.c (or gevent/gevent.c)
fixes the inf loop for psgi/coroae.