poll.c: always set revents, even if to zero

poll.c: always set revents, even if to zero

Match what is done to pfd[i].revents when compute_revents() returns
0 to the upstream gnulib's commit d42461c3 ("poll: fixes for large
fds", 2015-02-20).  The revents field is set to 0, without
incrementing the value rc to be returned from the function.  The
original code left the field to whatever random value the field was
initialized to.

This fixes occasional hangs in git-upload-pack on HPE NonStop.

Signed-off-by: Randall S. Becker <randall.becker@nexbridge.ca>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>