Fix incorrect reporting of error in Scanner.next() when Read reads > 0 bytes but returns os.EOF.

R=golang-dev, gri1, gri
CC=golang-dev
https://golang.org/cl/2083042

src/pkg/scanner/scanner.go

@@ -236,10 +236,12 @@ func (s *Scanner) next() int {
 				if s.srcEnd == 0 {
 					return EOF
 				}
+				if err != os.EOF {
 					s.error(err.String())
 					break
 				}
 			}
+		}
 		// at least one byte
 		ch = int(s.srcBuf[s.srcPos])
 		if ch >= utf8.RuneSelf {