Improve robustness when handling unexpected FunctionProto type locs.

Fixes an error when parsing Boost code (tuple_basic.hpp).

```
// Swallows any assignment   (by Doug Gregor)
namespace detail {

struct swallow_assign;
typedef void (detail::swallow_assign::*ignore_t)();
struct swallow_assign {
  swallow_assign(ignore_t(*)(ignore_t)) {}
  template<typename T>
  swallow_assign const& operator=(const T&) const {
    return *this;
  }
};

} // namespace detail

template<>
struct make_tuple_traits<detail::ignore_t(detail::ignore_t)> {
  typedef detail::swallow_assign type;
};
```

src/CppParser/Parser.cpp

@@ -1807,14 +1807,17 @@ Type* Parser::WalkType(clang::QualType QualType, clang::TypeLoc* TL,
         TypeLoc Next;
         if (TL && !TL->isNull())
         {
-            while (TL->getTypeLocClass() != TypeLoc::FunctionProto)
+            while (!TL->isNull() && TL->getTypeLocClass() != TypeLoc::FunctionProto)
             {
                 Next = TL->getNextTypeLoc();
                 TL = &Next;
             }
 
-            FTL = TL->getAs<FunctionProtoTypeLoc>();
-            RL = FTL.getReturnLoc();
+            if (!TL->isNull() && TL->getTypeLocClass() == TypeLoc::FunctionProto)
+            {
+                FTL = TL->getAs<FunctionProtoTypeLoc>();
+                RL = FTL.getReturnLoc();
+            }
         }
 
         auto F = new FunctionType();