iovec: make sure the caller actually wants anything in memcpy_fromiovecend

Based on upstream change 06ebb06d

One more instance when the caller requests 0 bytes instead of running
off and dereferencing potentially invalid iovecs.

Signed-off-by: Paul Lawrence <paullawrence@google.com>
Bug: 36279469
Change-Id: Ib8d529e17c07c77357ab70bd6a2d7e305d6b27f0