Put a nullptr check in Introspection code as this is crashing for me on OS X with optimized builds.
Put a nullptr check in Introspection code as this is crashing for me
on OS X with optimized builds.


Former-commit-id: 5eba8f63fbf8a4e0db23160d8247fe4176122c0d
darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide