do not crash on nil data value

R=r
DELTA=3  (3 added, 0 deleted, 0 changed)
OCL=34606
CL=34609

src/pkg/template/template.go

@@ -575,6 +575,9 @@ func (st *state) findVar(s string) reflect.Value {
 	for i := 0; i < len(elems); i++ {
 		// Look up field; data must be a struct.
 		data = reflect.Indirect(data);
+		if data == nil {
+			return nil
+		}
 		typ, ok := data.Type().(*reflect.StructType);
 		if !ok {
 			return nil