encoding/base64: remove ineffectual assignment in test

Change-Id: I4a0d5b2f76138895567939920fa5d83cbdec17d2
GitHub-Last-Rev: 061d9d1d5655a6a9d8371f08d2f77a0ed7a495cc
GitHub-Pull-Request: golang/go#30008
Reviewed-on: https://go-review.googlesource.com/c/160432Reviewed-by: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/encoding/base64/base64_test.go

@@ -175,7 +175,7 @@ func TestDecoder(t *testing.T) {
 		testEqual(t, "Read from %q = length %v, want %v", p.encoded, count, len(p.decoded))
 		testEqual(t, "Decoding of %q = %q, want %q", p.encoded, string(dbuf[0:count]), p.decoded)
 		if err != io.EOF {
-			count, err = decoder.Read(dbuf)
+			_, err = decoder.Read(dbuf)
 		}
 		testEqual(t, "Read from %q = %v, want %v", p.encoded, err, io.EOF)
 	}