Fix for out-of-bounds write in square test frame generator.

The length is set on construction and includes an assumption on the
image resolution, if the resolution changes, a square might be larger
than what fits into the buffer, causing an out of bounds write. This
CL fixes this simply by restricting the size of the square.

Bug: webrtc:11415
Change-Id: Iee14a1971997b4ae2fddef0a7af7c76a2509e879
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/170042
Commit-Queue: Sebastian Jansson <srte@webrtc.org>
Reviewed-by: Ali Tofigh <alito@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#30732}