This is slightly tidier than casting through function pointers. (Also
more defined? But we cast T* => void* within a function pointer all over
the place, so that's probably a lost cause.)
Change-Id: I8f435906f3066d1377eababf940e3db34c626acd
Reviewed-on: https://boringssl-review.googlesource.com/14313
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: Adam Langley <agl@google.com>
e94ec3f85b