Oclgrind reports out-of-bounds read in point source linker kernel

$ oclgrind --build-options -O0 python test_tree.py "test_extent_tree(cl._csc, 3)"
...
INFO:boxtree.tree:point source linking: complete

Invalid read of size 4 at global memory address 0x38000000061a80
	Kernel: box_point_sources
	Entity: Global(58,0,0) Local(58,0,0) Group(0,0,0)
	  %115 = load i32, i32 addrspace(1)* %arrayidx74, align 4, !dbg !89
	At line 79 of input.cl:
	  particle_id_t ps_start = tree_order_point_source_starts[s_start];