Allow wxCmdLineParser::Parse() to be called more than once.

Clear the previously collected parameters in the beginning to avoid
accumulating the results from several command lines.

Closes #16503.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@77499 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775