unpack-trees.c: work around run-time array initialization flaw on IRIX 6.5

unpack-trees.c: work around run-time array initialization flaw on IRIX 6.5

The c99 MIPSpro Compiler version 7.4.4m on IRIX 6.5 does not properly
initialize run-time initialized arrays.  An array which is initialized with
fewer elements than the length of the array should have the unitialized
elements initialized to zero.  This compiler only initializes the remaining
elements when the last element is a static parameter.  So work around it
by adding a "NULL" initialization parameter.

Signed-off-by: Brandon Casey <casey@nrlssc.navy.mil>
Signed-off-by: Junio C Hamano <gitster@pobox.com>