target_core_rd: break out unterminated loop during copy
The loop in rd_execute_rw() will never terminate if the
sg element has a zero size. Or it'll spill over into
outer space if the sg element is larger than the available
space.
So we need to add some safety catches here.
Cc: Nic Bellinger <nab@risingtidesystems.com> Signed-off-by: Hannes Reinecke <hare@suse.de> Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>