Oid: make sure not to dereference a NULL git_oid

Some calls like Reference.Target() can return NULL if the reference is
symbolic. Make sure newOidFromC() can handle these situations.

git.go

@@ -26,6 +26,10 @@ type Oid struct {
 }
 
 func newOidFromC(coid *C.git_oid) *Oid {
+	if coid == nil {
+		return nil
+	}
+
 	oid := new(Oid)
 	copy(oid.bytes[0:20], C.GoBytes(unsafe.Pointer(coid), 20))
 	return oid

reference_test.go

@@ -43,10 +43,22 @@ func TestRefModification(t *testing.T) {
 	checkFatal(t, err)
 	checkRefType(t, ref, SYMBOLIC)
 
+	if target := ref.Target(); target != nil {
+		t.Fatalf("Expected nil *Oid, got %v", target)
+	}
+
 	ref, err = ref.Resolve()
 	checkFatal(t, err)
 	checkRefType(t, ref, OID)
 
+	if target := ref.Target(); target == nil {
+		t.Fatalf("Expected valid target got nil")
+	}
+
+	if target := ref.SymbolicTarget(); target != "" {
+		t.Fatalf("Expected empty string, got %v", target)
+	}
+
 	if commitId.String() != ref.Target().String() {
 		t.Fatalf("Wrong ref target")
 	}