Initialize all fields of struct tm used by wxDateTime::Format().

Passing not fully initialized struct tm to strftime() results in Valgrind
errors and possible nastiness, see #12455.

Simply use memset() to set all fields of this system-dependent struct to 0
initially.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65494 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775