Improve error handling when looking for symbols with Options.CheckSymbols.

src/Generator/Generators/CSharp/CSharpTextTemplate.cs

@@ -2673,6 +2673,7 @@ namespace CppSharp.Generators.CSharp
                 NativeLibrary library;
                 Driver.Symbols.FindLibraryBySymbol(function.Mangled, out library);
 
+                if (library != null)
                     libName = Path.GetFileNameWithoutExtension(library.FileName);
             }
             if (libName != null && libName.Length > 3 && libName.StartsWith("lib"))