strings,bytes: use result of copy in subsequent slicing

This can get rid of a bounds check.
Followup to CL 622240.

Change-Id: I9d0a2c0408b8d274c46136d32d7a5fb09b4aad1c
Reviewed-on: https://go-review.googlesource.com/c/go/+/622955
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

src/runtime/string.go

@@ -51,8 +51,8 @@ func concatstrings(buf *tmpBuf, a []string) string {
 	}
 	s, b := rawstringtmp(buf, l)
 	for _, x := range a {
-		copy(b, x)
-		b = b[len(x):]
+		n := copy(b, x)
+		b = b[n:]
 	}
 	return s
 }