Clamp start of render request region
Clamp start of render request region

In extreme cases, it's possible for the start of a render request
to go past the actual length of the document.

Fixes #452

google/xi-editor,fuchsia-mirror/third_party-xi-editor,fuchsia-mirror/third_party-xi-editor,fuchsia-mirror/third_party-xi-editor,google/xi-editor,google/xi-editor,google/xi-editor,fuchsia-mirror/third_party-xi-editor