Fix segfault when calling tuple struct constructor as extern fn

Fixes #5315

src/test/run-pass/tuple-struct-constructor-pointer.rs

@@ -0,0 +1,19 @@
+// Copyright 2012 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 Foo(int);
+struct Bar(int, int);
+
+fn main() {
+    let f: extern fn(int) -> Foo = Foo;
+    let g: extern fn(int, int) -> Bar = Bar;
+    f(42);
+    g(4, 7);
+}