Fix crash in wxCharBuffer if memory allocation fails.

Handle memory allocation failure gracefully in wxCharTypeBuffer ctor.

Closes #15616.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@75092 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775