fmt, encoding/gob: fix misuse of Read

reader.Read() can return both 0,nil and len(buf),err.
To be safe, we use io.ReadFull instead of doing reader.Read directly.

Fixes #3472.

R=bradfitz, rsc, ality
CC=golang-dev
https://golang.org/cl/6285050

src/pkg/encoding/gob/decode.go

@@ -62,15 +62,15 @@ func overflow(name string) error {
 // Used only by the Decoder to read the message length.
 func decodeUintReader(r io.Reader, buf []byte) (x uint64, width int, err error) {
 	width = 1
-	_, err = r.Read(buf[0:width])
-	if err != nil {
+	n, err := io.ReadFull(r, buf[0:width])
+	if n == 0 {
 		return
 	}
 	b := buf[0]
 	if b <= 0x7f {
 		return uint64(b), width, nil
 	}
-	n := -int(int8(b))
+	n = -int(int8(b))
 	if n > uint64Size {
 		err = errBadUint
 		return

src/pkg/fmt/scan.go

@@ -337,7 +337,10 @@ func (r *readRune) readByte() (b byte, err error) {
 		r.pending--
 		return
 	}
-	_, err = r.reader.Read(r.pendBuf[0:1])
+	n, err := io.ReadFull(r.reader, r.pendBuf[0:1])
+	if n != 1 {
+		return 0, err
+	}
 	return r.pendBuf[0], err
 }