base64: refactor encoder.Write

"nn" can never be zero for any input "p", so no check is needed.
This change should improve readability a bit.

LGTM=nigeltao
R=golang-codereviews, bradfitz, nigeltao
CC=golang-codereviews
https://golang.org/cl/76610045

src/pkg/encoding/base64/base64.go

@@ -159,14 +159,12 @@ func (e *encoder) Write(p []byte) (n int, err error) {
 		nn := len(e.out) / 4 * 3
 		if nn > len(p) {
 			nn = len(p)
-		}
 			nn -= nn % 3
-		if nn > 0 {
+		}
 		e.enc.Encode(e.out[0:], p[0:nn])
 		if _, e.err = e.w.Write(e.out[0 : nn/3*4]); e.err != nil {
 			return n, e.err
 		}
-		}
 		n += nn
 		p = p[nn:]
 	}