openssh: fix potential signed overflow to enable compilation with -ftrapv

openssh: fix potential signed overflow to enable compilation with -ftrapv

Pointer arithmatic results in implementation defined signed integer
type, so that 's - src' in strlcpy and others may trigger signed overflow.
In case of compilation by gcc or clang with -ftrapv option, the overflow
would lead to program abort.

Upstream-status: Submitted [https://bugzilla.mindrot.org/show_bug.cgi?id=2608]

Signed-off-by: Yuanjie Huang <yuanjie.huang@windriver.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>