fold checks into iterate_and_advance()

they are open-coded in all users except iov_iter_advance(), and there
they wouldn't be a bad idea either - as it is, iov_iter_advance(i, 0)
ends up dereferencing potentially past the end of iovec array.  It
doesn't do anything with the value it reads, and very unlikely to
trigger an oops on dereference, but it is not impossible.

Reported-by: Jiri Slaby <jslaby@suse.cz>
Reported-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>