Fix minor issues with updating go-mode in an empty buffer

	Fix minor issues with updating go-mode in an empty buffer

  Sometimes during editing, when the buffer is emptied (for example,
  as part of a call to M-x gofmt), an error is raised when
  remove-text-properties is called. Ensure that these calls use point-max
  instead of buffer-size to determine their end point.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/189042