Add Lingzhu Xiang to AUTHORS
CS

Signed-off-by: Sebastian Ramacher <sebastian+dev@ramacher.at>
Correct the given scale value
Move input validation back to document_open

Re-setting scale back to 1 does not make sense if the change to scale comes from
zooming in zathura.

Signed-off-by: Sebastian Ramacher <sebastian+dev@ramacher.at>
Do not adjust scale if given scale is not positive

Signed-off-by: Sebastian Ramacher <sebastian+dev@ramacher.at>
CS

Signed-off-by: Sebastian Ramacher <sebastian+dev@ramacher.at>