image/jpeg: when following component selectors, only consider valid

components.

This fixes decoding JPEG images where the component selector is 0. Such
images are rare, but not impossible.

Change-Id: I6d221bce01cce8cc0440e117543233371782ca22
Reviewed-on: https://go-review.googlesource.com/6421Reviewed-by: Rob Pike <r@golang.org>

src/image/jpeg/scan.go

@@ -66,7 +66,7 @@ func (d *decoder) processSOS(n int) error {
 	for i := 0; i < nComp; i++ {
 		cs := d.tmp[1+2*i] // Component selector.
 		compIndex := -1
-		for j, comp := range d.comp {
+		for j, comp := range d.comp[:d.nComp] {
 			if cs == comp.c {
 				compIndex = j
 			}