mesa: Mark render-to-texture as unsafe if there's no pipe_resource

Let's be slightly more defensive here.  If a texture image doesn't have
an associated pipe_resource allocated, then render_texture() will pass
that along to _mesa_update_renderbuffer_surface(), which will crash on a
NULL pointer dereference.  So, if there isn't a pipe_resource, then we
should just skip this altogteher.

Today, this isn't an issue, because each gl_texture_image always
allocates a pipe_resource up front.  On a branch of mine, I prototyped
some improvements to the compressed texture fallback handling, where it
would defer resource allocation, examine the source image's block data,
and dynamically select a format based on that, then allocate it later.

With that prototype in place, we saw crashes the Android "My Talking
Tom" series of games, which appear to be attaching ASTC textures to a
framebuffer color attachment.  That FBO would be incomplete anyway, as
ASTC textures aren't renderable, but we got into a situation where the
render-to-texture code was crashing due to the lack of pt before it
could properly signal that it was incomplete and bailing.

Technically, we don't need this now, but I figure that being defensive
won't hurt and this would probably save whoever encounters such an issue
in the future a bunch of frustrating debugging.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/17508>

src/mesa/main/fbobject.c

@@ -512,6 +512,7 @@ driver_RenderTexture_is_safe(const struct gl_renderbuffer_attachment *att)
       att->Texture->Image[att->CubeMapFace][att->TextureLevel];
 
    if (!texImage ||
+       !texImage->pt ||
        texImage->Width == 0 || texImage->Height == 0 || texImage->Depth == 0)
       return false;