Fix select decoding with bogus (huge or negative) nfds.

Fix select decoding with bogus (huge or negative) nfds.

We used to allocate and fetch bit arrays using a sanitized
length, but then iterate over them with "j < arg[0]" condition,
where arg[0] is not sanitized. This segfaults if arg[0] is huge
or negative. This change fixes this.

Add test/select.c to capture the case.

Signed-off-by: Dr. David Alan Gilbert <dave@treblig.org>
Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>