Correct wxConvAuto::ToWChar() behaviour with wxNO_LEN input size.

Correct wxConvAuto::ToWChar() behaviour with wxNO_LEN input size.

We didn't handle the case when the length of the input buffer was not
specified correctly and wxConvAuto::DetectBOM() could read beyond the end of
input. Moreover, the unit test actually relied on this as it didn't pass the
correct length for the literal strings with embedded NULs. This somehow worked
with MSVC but failed with MinGW (see #10713).

Correct the code to handle wxNO_LEN case correctly and fix the unit test to
pass the correct lengths.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65739 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775