Capture/Replay: Fix CaptureShaderSource_string. Was accessing a buffer out-of-bounds. Also wasn't handling negative length. Test: ASAN build Bug: angleproject:3611 Change-Id: I0f1cfa09d0488de20928d90e910803d98c225968 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1796082 Reviewed-by: Jonah Ryan-Davis <jonahr@google.com> Commit-Queue: Jamie Madill <jmadill@chromium.org>