Fix one more nullptr.
Added a stream compaction test case
Only run tutorial lesson 12 if it can find libOpenCL
Bail from tutorial on first error
Remove -g from static test runs.

It fails on OS X when the object file was generated with trunk llvm.
Generating the binary is fine, but building the dSYM folder fails
with assertion:

Assertion failed: (linked_addr_pos != line_table_map.end()), function FixReferences, file /SourceCache/dwarf_utilities/dwarf_utilities-119/source/DWARFdSYM.cpp, line 3749.

I'm hoping this problem will magically fix itself with either an llvm or
dsymutil update.
Disable nightly opencl testing on os x

My mac mini has flaky AMD drivers, and the CPU drivers only support
1D work groups.
Fixed generation of overflowing constants in Int(32) expressions.
Merge branch 'master' of https://github.com/halide/Halide
Remove C++11ism
Merge branch 'master' into cse_rewrite