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

interface/wx/base64.h

@@ -106,7 +106,8 @@ size_t wxBase64EncodedSize(size_t len);
     This overload is a raw decoding function and decodes the data into the
     provided buffer @a dst of the given size @e dstLen. An error is returned if
     the buffer is not large enough -- that is not at least
-    wxBase64DecodedSize(srcLen) bytes.
+    wxBase64DecodedSize(srcLen) bytes. Notice that the buffer will @e not be
+    @NULL-terminated.
 
     This overload returns the number of bytes written to the buffer or the
     necessary buffer size if @a dst was @NULL or @c wxCONV_FAILED on error,