libminc:vsnprintf add support for NULL destination pointer.
libminc:vsnprintf add support for NULL destination pointer.

-Add support for returning the amount of characters that would have been
written if the buffer was large enough.
-Protect code against NULL dereference.

Change-Id: Ifb2041f4757e8a99f255d94768ba19621bc0ea16

http://gerrit.minix3.org/#/c/2560/

veritas-shine/minix3-rpi,veritas-shine/minix3-rpi,veritas-shine/minix3-rpi,veritas-shine/minix3-rpi,veritas-shine/minix3-rpi,veritas-shine/minix3-rpi,veritas-shine/minix3-rpi,veritas-shine/minix3-rpi