CAMediaTimingFunctionBuiltin is a class in QuartzCore. Its initWithCoder: method
reads an Int "index" then passes that to builtin_function
```
mov ebx, edi <-- controlled unsigned int
mov r14d, ebx
lea r15, __ZL9functions_0 ; functions
mov rax, [r15+r14*8]
```
if rax is non-null it's returned as an objective-c object pointer and the objective-c retain
selector is sent to it.
Serialized poc in attached file with an index of 12345678.
tested on MacOS 10.12.3 (16D32)
[functionbuiltin.zip](https://bugs.chromium.org/p/project-zero/issues/attachment?aid=274580)
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