If the function returns early due to an error, then the coordinates of the
result will have their |top| value set to a value beyond what has actually
been been written. Fix that, and make it easier to avoid such issues in the
future by refactoring the code.
As a bonus, avoid a false positive MSVC 64-bit opt build "potentially
uninitialized value used" warning.
Change-Id: I8c48deb63163a27f739c8797962414f8ca2588cd
Reviewed-on: https://boringssl-review.googlesource.com/6579
Reviewed-by: David Benjamin <davidben@google.com>