Avoid erl_lint internal errors for badly formed export_type declarations

[Kostis Sagonas]

Avoid errors for a badly formed export_type declarations

In the following program, erl_lint crashed with an erl_lint internal 
error. With this patch it does not, but prints "bad export_type
declaration" errors instead.

-module(baz).
-export([test/0]).

-export_type(t/0).
-export_type([3.14]).

-type t() :: any().

test() -> 42.


I have not added the program in the erl_lint tests (somebody might want 
to do that) but the patch that fixes this is here:

   git fetch git://github.com/kostis/otp.git bad-export_type

Kostis