don't crash when there are multiple conflicting implementations of Drop

Fixes rust-lang#28568

Author: arielb1

src/librustc/middle/ty/mod.rs

@@ -1695,7 +1695,6 @@ impl<'tcx, 'container> AdtDefData<'tcx, 'container> {
     }
 
     pub fn set_destructor(&self, dtor: DefId) {
-        assert!(self.destructor.get().is_none());
         self.destructor.set(Some(dtor));
     }
 

src/test/compile-fail/issue-28568.rs

@@ -0,0 +1,23 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+struct MyStruct;
+
+impl Drop for MyStruct {
+//~^ ERROR conflicting implementations for trait
+    fn drop(&mut self) { }
+}
+
+impl Drop for MyStruct {
+//~^ NOTE conflicting implementation here
+    fn drop(&mut self) { }
+}
+
+fn main() {}