syscallcompat: OpenNofollow: fix relPath="" case

Sometimes want to open baseDir itself. This case
was broken, fix it.

internal/syscallcompat/open_nofollow.go

@@ -27,6 +27,10 @@ func OpenNofollow(baseDir string, relPath string, flags int, mode uint32) (fd in
 	if err != nil {
 		return -1, err
 	}
+	// Caller wanted to open baseDir itself?
+	if relPath == "" {
+		return dirfd, nil
+	}
 	// Split the path into components and separate intermediate directories
 	// and the final basename
 	parts := strings.Split(relPath, "/")

internal/syscallcompat/open_nofollow_test.go

@@ -34,4 +34,11 @@ func TestOpenNofollow(t *testing.T) {
 	if err != syscall.ELOOP {
 		t.Errorf("expected ELOOP, got %v", err)
 	}
+	// Check to see that the base dir can be opened as well
+	fd, err = OpenNofollow(tmpDir, "", syscall.O_RDONLY, 0)
+	if err != nil {
+		t.Errorf("cannot open base dir: %v", err)
+	} else {
+		syscall.Close(fd)
+	}
 }