Add missing initializer for RasterizerStorageGLES3::Shader::ubo_size

drivers/gles3/rasterizer_storage_gles3.h

@@ -425,6 +425,7 @@ public:
 			: dirty_list(this) {
 
 			shader = NULL;
+			ubo_size = 0;
 			valid = false;
 			custom_code_id = 0;
 			version = 1;