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

docs/changes.txt

@@ -371,6 +371,7 @@ All:
  * wxLog::DoLogRecord() has access to the location of the log message
    (file, line and function name) and id of the thread which generated it.
  * SetThreadActiveTarget() allows to set up thread-specific log targets.
+- Fix output buffer overflow in wxBase64Decode() (Eric W. Savage).
 
 All (GUI):