image/jpeg: for progressive JPEGs, the first SOS segment doesn't

necessarily contain all components.

Fixes #4975.

R=r, minux.ma, bradfitz
CC=golang-dev
https://golang.org/cl/7469043

src/pkg/image/jpeg/scan.go

@@ -109,9 +109,11 @@ func (d *decoder) processSOS(n int) error {
 	myy := (d.height + 8*v0 - 1) / (8 * v0)
 	if d.img1 == nil && d.img3 == nil {
 		d.makeImg(h0, v0, mxx, myy)
+	}
 	if d.progressive {
 		for i := 0; i < nComp; i++ {
 			compIndex := scan[i].compIndex
+			if d.progCoeffs[compIndex] == nil {
 				d.progCoeffs[compIndex] = make([]block, mxx*myy*d.comp[compIndex].h*d.comp[compIndex].v)
 			}
 		}