x/image/tiff: fix division by zero when decoding empty image

Return a zero width or zero height image of the appropriate type.

Fixes golang/go#10393.

Change-Id: I3aa7b6125447726600fb072ce1e8682373ce2a57
Reviewed-on: https://go-review.googlesource.com/9182
Reviewed-by: Nigel Tao <nigeltao@golang.org>
2 files changed