Fix code to use BaseClass property instead of first base.

The first base could be ignored, BaseClass does the right thing.

src/AST/ClassExtensions.cs

@@ -157,7 +157,7 @@ namespace CppSharp.AST
             Class @base = null;
 
             if (@class.HasBaseClass)
-                @base = @class.Bases[0].Class;
+                @base = @class.BaseClass;
 
             return @base?.IsRefType == true && @base.IsGenerated;
         }

src/Generator/Generators/C/CppHeaders.cs

@@ -496,7 +496,7 @@ namespace CppSharp.Generators.Cpp
                 Write(" abstract sealed");
 
             if (!@class.IsStatic && @class.HasRefBase())
-                Write($" : public {QualifiedIdentifier(@class.Bases[0].Class)}");
+                Write($" : public {QualifiedIdentifier(@class.BaseClass)}");
         }
 
         public void GenerateClassProperties(Class @class)