Fix bug when copying slice to itself

If proptools.CopyProperties is passed two values that point same
slice then setting the destination slice to a new slice will
overwrite the source slice, and the properties struct that is both
the source and destination will have an empty slice.  Copy into
the new slice using a new reflect.Value, and then update the
destination.

Change-Id: I1bfcdc51e4278ea7c7ed81dafc928a5471219f05

proptools/proptools.go

@@ -64,10 +64,12 @@ func CopyProperties(dstValue, srcValue reflect.Value) {
 					panic(fmt.Errorf("can't copy field %q: slice elements are "+
 						"not strings", field.Name))
 				}
-				newSlice := reflect.MakeSlice(field.Type, srcFieldValue.Len(),
-					srcFieldValue.Len())
-				dstFieldValue.Set(newSlice)
-				reflect.Copy(dstFieldValue, srcFieldValue)
+				if srcFieldValue != dstFieldValue {
+					newSlice := reflect.MakeSlice(field.Type, srcFieldValue.Len(),
+						srcFieldValue.Len())
+					reflect.Copy(newSlice, srcFieldValue)
+					dstFieldValue.Set(newSlice)
+				}
 			} else {
 				dstFieldValue.Set(srcFieldValue)
 			}