src/internal/bytealg: fix indexing error in Compare()

Fixes #2139

src/internal/bytealg/bytealg.go

@@ -15,9 +15,9 @@ func Compare(a, b []byte) int {
 	// Compare for differing bytes.
 	for i := 0; i < len(a) && i < len(b); i++ {
 		switch {
-		case a[0] < b[0]:
+		case a[i] < b[i]:
 			return -1
-		case a[0] > b[0]:
+		case a[i] > b[i]:
 			return 1
 		}
 	}