[auxiliary.c] for degree 1, hard-code value of alpha in get_alpha()
[auxiliary.c] for degree 1, hard-code value of alpha in get_alpha()

This fixes an annoying bug with:
./factor.sh 4989987986917107645320509935679224561541664588721574446483221849128942141
where one calls get_alpha() with a degree-1 polynomial which is identically
zero mod 109.

mancoast/cado-nfs,mancoast/cado-nfs,mancoast/cado-nfs,mancoast/cado-nfs,mancoast/cado-nfs,mancoast/cado-nfs,mancoast/cado-nfs