Fix generation of C++ constructor for abstract classes.

src/Generator/Generators/C/CppSources.cs

@@ -369,13 +369,16 @@ namespace CppSharp.Generators.Cpp
 
             if (@class.IsRefType)
             {
-                if (method.IsConstructor && !@class.IsAbstract)
+                if (method.IsConstructor)
+                {
+                    if (!@class.IsAbstract)
                     {
                         var @params = GenerateFunctionParamsMarshal(method.Parameters, method);
                         Write($"{Helpers.InstanceIdentifier} = new ::{method.Namespace.QualifiedOriginalName}(");
                         GenerateFunctionParams(@params);
                         WriteLine(");");
                     }
+                }
                 else
                 {
                     GenerateFunctionCall(method);