mesa/objectlabel: don't return info on genned but never bound textures.

This fixes some cases in the CTS KHR debug tests where it uses
glIsTexture to find an invalid ID and then call GetObjectLabel.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>

src/mesa/main/objectlabel.c

@@ -196,7 +196,7 @@ get_label_pointer(struct gl_context *ctx, GLenum identifier, GLuint name,
    case GL_TEXTURE:
       {
          struct gl_texture_object *texObj = _mesa_lookup_texture(ctx, name);
-         if (texObj)
+         if (texObj && texObj->Target)
             labelPtr = &texObj->Label;
       }
       break;