Fix auto test failure output on QNX

On QNX mbstowcs does not ignore last parameter (max) when first (dest)
is NULL. Set it to sufficiently large value to yield proper results on
QNX. Other platforms (standard libraries) will ignore this value anyway.

Change-Id: Ie4695254d45082e151a052bf16de684af3b1ba1e
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>