Add some null asserts, closes #1134

Adding `assert(object_type);` silences a null-dereference warning when compiling cppfront on GCC 14 using -Werror=nonnull -O2 (both switches together)

Running all regression tests doesn't fire the assertion, but it's good hygiene to have it there anyway. GCC could be diagnosing a real error that I don't see, and that happens not to be exercised by the regression tests. At least now with the assertion we'll get a clear ICE to track down, if we ever do encounter a source file where it does fire.

source/sema.h

@@ -1518,9 +1518,13 @@ class sema
     auto check(parameter_declaration_node const& n)
         -> bool
     {
+        assert(n.declaration);
+
         auto type_name = std::string{};
         if (n.declaration->has_declared_return_type()) {
-            type_name = n.declaration->get_object_type()->to_string();
+            auto object_type = n.declaration->get_object_type();
+            assert(object_type);
+            type_name = object_type->to_string();
         }
 
         if (