Never overflow the output buffer in wxBase64Decode().

Never overflow the output buffer in wxBase64Decode().

Don't write extra NUL bytes obtained by decoding the padding at the end of
input into the output buffer as there may be not enough place in it for them.
And in any case the buffer is not (always) NUL-terminated as no NUL bytes are
obtained in absence of padding, so it's better to never terminate it for
consistency.

Closes #11101.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@61753 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775