gpg: Make sure a user ID packet has always a terminating Nul in memory.

* g10/keygen.c (write_uid): Avoid overflow.
--

Also the actual length if the user ID is given by LEN, using NAME
diretcly is often more convenient.

g10/packet.h

@@ -293,7 +293,7 @@ typedef struct
   } flags;
   /* The text contained in the user id packet, which is normally the
      name and email address of the key holder (See RFC 4880 5.11).
-     (Serialized.)  */
+     (Serialized.). For convenience an extra Nul is always appended.  */
   char name[1];
 } PKT_user_id;