Fix ICE when calling non-functions within closures

The visitor for walking function bodies did not previously properly
handle error-cases for function calls. These are now ignored,
preventing the panic.

src/test/compile-fail/issue-46771.rs

@@ -0,0 +1,14 @@
+// Copyright 2017 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.
+
+fn main() {
+    struct Foo;
+    (1 .. 2).find(|_| Foo(0) == 0); //~ ERROR expected function, found `main::Foo`
+}