9$: Warn on semicolon in if-else chain

[strager]

The following code likely has a mistake:

if (x) {
  f();
} else; {  // mistake: stray semicolon
  g();
}

produce a warning for code like this.

[strager]

@someoneigna made a PR for this task. (GitHub won't let me assign them this task, though, because they haven't made a comment.) #290