Fix possible segv in Function shared proxy

Summary:
[Folly] Fix possible segv in `Function` shared proxy when invoking the (public!) `nullptr`-taking ctor.

This proxy is used when converting `folly::Function` to `std::function` with shared state.

Reviewed By: ot

Differential Revision: D18517365

fbshipit-source-id: c414c1d1ae2b4f30ffa3269b216be07fedbf838c

folly/Function.h

@@ -330,6 +330,9 @@ class FunctionTraitsSharedProxy {
   explicit FunctionTraitsSharedProxy(Function<F>&& func)
       : sp_(std::make_shared<Function<F>>(std::move(func))) {}
   R operator()(A&&... args) const {
+    if (!sp_) {
+      throw_exception<std::bad_function_call>();
+    }
     return (*sp_)(static_cast<A&&>(args)...);
   }
 

folly/test/FunctionTest.cpp

@@ -1092,6 +1092,11 @@ TEST(Function, asSharedProxy_args_const) {
   EXPECT_EQ(562, spcopy(5, 6));
 }
 
+TEST(Function, asSharedProxy_nullptr) {
+  auto sp = folly::Function<int(int, int) const>::SharedProxy(nullptr);
+  EXPECT_THROW(sp(3, 4), std::bad_function_call);
+}
+
 TEST(Function, NoAllocatedMemoryAfterMove) {
   Functor<int, 100> foo;