Improve strncpy_s src/dst overlap check

Improve strncpy_s src/dst overlap check

Let m = user estimate of the (max) src string length, low = smaller
address of (src, dst), hi = larger address (src, dst).

if (low + (m - 1) >= hi), we have a *potential* overlapping copy which
is not allowed. Before we declare overlap - and return an error -
retry the check with m = actual src string length.

The resulting "test string" failure affected aarch64 (only) because of
differences in test code stack variable placement / alignment.

Change-Id: I2931d1ce2c61af3d3880075b033d2a4c4e421f09
Signed-off-by: Dave Barach <dave@barachs.net>