GPU: Fix Segmentation Fault Freeing Failed Shader

The CPP Shader class does not initialize the interface attribute.
What will crash when deleting the shader.

Reviewed By: Clément Foucault

Differential Revision: https://developer.blender.org/D8740

source/blender/gpu/intern/gpu_shader_private.hh

@@ -36,7 +36,7 @@ namespace gpu {
 class Shader {
  public:
   /** Uniform & attribute locations for shader. */
-  ShaderInterface *interface;
+  ShaderInterface *interface = nullptr;
 
  protected:
   /** For debugging purpose. */