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.