mesa: fix incorrect error in _mesa_BindTextureUnit()

If the texture object exists, but the Name field is zero, it means
the object was created but never bound to a target.  Trying to bind it
in _mesa_BindTextureUnit() should generate GL_INVALID_OPERATION.

Fixes piglit's arb_direct_state_access-bind-texture-unit test.

Reviewed-by: Tapani Pälli <tapani.palli@intel.com>

src/mesa/main/texobj.c

@@ -1816,7 +1816,8 @@ _mesa_BindTextureUnit(GLuint unit, GLuint texture)
       return;
    }
    if (texObj->Target == 0) {
-      _mesa_error(ctx, GL_INVALID_ENUM, "glBindTextureUnit(target)");
+      /* Texture object was gen'd but never bound so the target is not set */
+      _mesa_error(ctx, GL_INVALID_OPERATION, "glBindTextureUnit(target)");
       return;
    }
    assert(valid_texture_object(texObj));