Clipboard fixes

[matt335672]

Fixes #1992

There are a few other improvements in this area too, all as separate commits. In order:-

• The struct clip_file_desc in clipboard_common.h is meant to hold the data sent in a CLIPRDR_FILEDESCRIPTOR. The filename is 260 Unicode characters. Since each filename could occupy up to 4 bytes, the allocated space for the cFileName member is too small.
• (See clipboard file: Odd behaviour when transferring files client to server direction #1992). The logic to skip the padding at the end of a CLIPRDR_FILEDESCRIPTOR is wrong, and only works for simple (i.e. European) filenames.
• An unnecessary malloc/free pair is removed by using a stack variable instead.
• A check is added that the buffer containing a CLIPRDR_FILEDESCRIPTOR has enough data before the structure is read.

[metalefty]

I confirmed it fixes #1992.

[metalefty]

LGTM, thanks for the quick fix!