Fix the size of the buffer when using wxIPC::Execute() with DDE.

Don't multiply the "realSize" variable by sizeof(wxChar) when passing it to
DdeClientTransaction(), as its name indicates it is already supposed to be the
size of the data and not the length of the string so just ensure that it is
always correctly set to the size and not length when initializing it.

Closes #13734.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@69943 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775