Fixed a crash when having a secondary base which is not a class.

It can be a template parameter, for instance.

Signed-off-by: Dimitar Dobrev <dpldobrev@protonmail.com>

src/Generator/Passes/MultipleInheritancePass.cs

@@ -46,7 +46,7 @@ namespace CppSharp.Passes
             {
                 var @base = @class.Bases[i];
                 var baseClass = @base.Class;
-                if (baseClass.IsInterface) continue;
+                if (baseClass == null || baseClass.IsInterface) continue;
 
                 var @interface = GetInterface(baseClass);
                 @class.Bases[i] = new BaseClassSpecifier(@base) { Type = new TagType(@interface) };

tests/CSharp/CSharp.h

@@ -924,3 +924,8 @@ public:
     int conflictWithProperty();
     int getConflictWithProperty();
 };
+
+template <class Category, class Traversal>
+struct iterator_category_with_traversal : Category, Traversal
+{
+};