common: Use larger buffer for homedir in case of 64 bit UIDs.

* common/homedir.c (_gnupg_socketdir_internal): Enlarge PREFIX by 6
bytes for "/gnupg".

The temporary buffer was to short for the extra "/gnupg". However the
20 bytes for the UID is large enough for all 32 bit UIDs and would
only fail (detected) if a 64 bit UID is used.

• Fixes-commit: 17efcd2a2acdc3b7f00711272aa51e5be2476921
• Reported-by: Rainer Perske.
• Signed-off-by: Werner Koch <wk@gnupg.org>