Regression: runtime crash in test/opengl/ JIT tests

[abestephensg]

I'm seeing this crash in most of the test/opengl/ JIT tests:
#0 0x0000000104beb082 in 0x104beb082 ()
#1 0x00000001077e515c in 0x1077e515c ()
#2 0x0000000104beee28 in 0x104beee28 ()
#3 0x0000000104bef3e7 in 0x104bef3e7 ()
#4 0x0000000100210e4a in halide_copy_to_host at /Users/abstephens/Halide-master/src/DeviceInterface.cpp:57
#5 0x00000001000bb6a7 in Halide::Buffer::copy_to_host() at /Users/abstephens/Halide-master/src/Buffer.cpp:238
#6 0x00000001002be145 in Halide::ImageBase::copy_to_host() at /Users/abstephens/Halide-master/src/Image.cpp:111
#7 0x0000000100004f34 in void Halide::Func::realize(Halide::Image, Halide::Target const&) at /Users/abstephens/Halide-master/build-xcode/include/Halide.h:7809
#8 0x0000000100002229 in main at /Users/abstephens/Halide-master/test/opengl/copy_to_device.cpp:38

@abadams regressed it to 3dfcc41

[abestephensg]

Cannot reproduce in a7c3433