Intermittent CI failure in wrench.

[glennw]

OpenGL version 3.3 (Core Profile) Mesa 12.0.1, Gallium 0.4 on softpipe
hidpi factor: 1 (native 1)
Shader override path: None
thread 'main' panicked at 'index out of bounds: the len is 0 but the index is 1844674434858745856', src/main.rs:87
note: Run with `RUST_BACKTRACE=1` for a backtrace.

Traceback (most recent call last):
  File "run_tests.py", line 64, in <module>
    subprocess.check_call(['../target/release/wrench

[glennw]

cc @vvuk

This is in the percentile calculate function.