Fix NULL pointer reference in _outPathTarget()

The array sortgrouprefs[] inside PathTarget might be NULL if we have not
identified sort/group columns in this tlist. In that case we would have
a NULL pointer reference in _outPathTarget() when trying to print
sortgrouprefs[] with WRITE_INDEX_ARRAY as we are using the length of
PathTarget->exprs as its array length.

Attached is a fix that can address this problem.

Thanks
Richard