[release-branch.go1.4] image/jpeg: handle Read returning n > 0, err != nil in d.fill

««« CL 178120043 / 95f5614b4648
image/jpeg: handle Read returning n > 0, err != nil in d.fill

Fixes issue 9127.

LGTM=r
R=bradfitz, r
CC=golang-codereviews, nigeltao
https://codereview.appspot.com/178120043
»»»

TBR=r
CC=golang-codereviews
https://codereview.appspot.com/181870043