bytes: return early in Repeat if count is 0

This matches the implementation of strings.Repeat and slightly increases
performance:

name      old time/op  new time/op  delta
Repeat-8   145ns ±12%   125ns ±29%  -13.35%  (p=0.009 n=10+10)

Change-Id: Ic0a0e2ea9e36591286a49def320ddb67fe0b2c50
Reviewed-on: https://go-review.googlesource.com/c/go/+/166399
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/bytes/bytes.go

@@ -499,13 +499,16 @@ func Map(mapping func(r rune) rune, s []byte) []byte {
 // It panics if count is negative or if
 // the result of (len(b) * count) overflows.
 func Repeat(b []byte, count int) []byte {
+	if count == 0 {
+		return []byte{}
+	}
 	// Since we cannot return an error on overflow,
 	// we should panic if the repeat will generate
 	// an overflow.
 	// See Issue golang.org/issue/16237.
 	if count < 0 {
 		panic("bytes: negative Repeat count")
-	} else if count > 0 && len(b)*count/count != len(b) {
+	} else if len(b)*count/count != len(b) {
 		panic("bytes: Repeat count causes overflow")
 	}