Fix panic on backspace at start of buffer
Fix panic on backspace at start of buffer

It looks like there was a guard for this case, but it was overly
restrictive, only effective for empty buffers.

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