fmt: fix caching bug in Scan

Black box test is too time-consuming, as the bug
does not appear until Scan has processed 2 GB of
input in total across multiple calls, so no test.

Thanks to Frederick Mayle for the diagnosis and fix.

Fixes #2809.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5611043

src/pkg/fmt/scan.go

@@ -366,6 +366,7 @@ func newScanState(r io.Reader, nlIsSpace, nlIsEnd bool) (s *ss, old ssave) {
 	s.fieldLimit = hugeWid
 	s.maxWid = hugeWid
 	s.validSave = true
+	s.count = 0
 	return
 }