Fixed: When selector is only a ':' in a send message statement the new compiler crashed.

The program below crashed on the last line with a 'null is not an object' error message. Also added a test case.

```objjc
@implementation MyObject {
- (int):(int)a {
  return a;
}
@end

var a = [[MyObject alloc] init];
var b = [a:3];
```