opencrypto: Handle end-of-cursor conditions in crypto_cursor_segment()

Some consumers, e.g., swcr_encdec(), may call crypto_cursor_segment()
after having advanced the cursor to the end of the buffer. In this case
I believe the right behaviour is to return NULL and a length of 0.

When this occurs with a CRYPTO_BUF_VMPAGE buffer, the cc_vmpage pointer
will point past the end of the page pointer array, so
crypto_cursor_segment() ends up dereferencing a random pointer before
the function returns a length of 0. The uio-backed cursor has
a similar problem.

Address this by keeping track of the residual buffer length and
returning immediately once the length is zero.

PR: 271766
Reported by: Andrew "RhodiumToad" Gierth <andrew@tao11.riddles.org.uk>
Reviewed by: jhb
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D40428