mesa: add missing error-path

The ARB_shader_objects spec says the following:

> The error INVALID_VALUE is generated by any command that takes one or
> more handles as input, and one or more of these handles are not an
> object handle generated by OpenGL.

And a long, long time ago, we used do to just that for
glDeleteObjectARB... Until 9ac9605de1, all the way back in February 2006,
where the error condition was removed without explanation.

Let's restore it, because it should really be there.

This was noticed by running the tests that are in the mesa-demos
repository, that actually tested this condition.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Cc: mesa-stable
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16211>

src/mesa/main/shaderapi.c

@@ -1705,7 +1705,7 @@ _mesa_DeleteObjectARB(GLhandleARB obj)
          delete_shader(ctx, obj);
       }
       else {
-         /* error? */
+         _mesa_error(ctx, GL_INVALID_VALUE, "glDeleteObjectARB");
       }
    }
 }