don't lock if ref is nil

Ensure that d.mu is only dereferenced if d is not-nil, to avoid a null
dereference panic.

client/allocdir/alloc_dir.go

@@ -117,12 +117,13 @@ func NewAllocDir(logger hclog.Logger, allocDir string) *AllocDir {
 // Copy an AllocDir and all of its TaskDirs. Returns nil if AllocDir is
 // nil.
 func (d *AllocDir) Copy() *AllocDir {
-	d.mu.RLock()
-	defer d.mu.RUnlock()
-
 	if d == nil {
 		return nil
 	}
+
+	d.mu.RLock()
+	defer d.mu.RUnlock()
+
 	dcopy := &AllocDir{
 		AllocDir:  d.AllocDir,
 		SharedDir: d.SharedDir,