bytes: add a boundary test for Count of 1 byte

This bottoms out in internal/bytealg.Count, which does a fair amount
of avx shenanigans. Make sure we're not reading out of bounds.

Change-Id: Ied0e461f536f75acc24c6797f7fc74e3f3901c10
Reviewed-on: https://go-review.googlesource.com/c/go/+/533116
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Jorropo <jorropo.pgm@gmail.com>
Reviewed-by: Achille Roussel <achille.roussel@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>

src/bytes/boundary_test.go

@@ -98,3 +98,18 @@ func TestIndexNearPageBoundary(t *testing.T) {
 	}
 	q[len(q)-1] = 0
 }
+
+func TestCountNearPageBoundary(t *testing.T) {
+	t.Parallel()
+	b := dangerousSlice(t)
+	for i := range b {
+		c := Count(b[i:], []byte{1})
+		if c != 0 {
+			t.Fatalf("Count(b[%d:], {1})=%d, want 0\n", i, c)
+		}
+		c = Count(b[:i], []byte{0})
+		if c != i {
+			t.Fatalf("Count(b[:%d], {0})=%d, want %d\n", i, c, i)
+		}
+	}
+}