st/mesa: don't dereference stObj->pt if NULL

It seems a user app can get us into this state, I trigger the fail
running fbo-maxsize inside virgl, it fails to create the backing
storage for the texture object, but then segfaults here when it
should fail the completeness test.

Cc: "9.2" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Brian Paul <brianp@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

src/mesa/state_tracker/st_cb_fbo.c

@@ -502,7 +502,7 @@ st_validate_attachment(struct gl_context *ctx,
    if (att->Type != GL_TEXTURE)
       return GL_TRUE;
 
-   if (!stObj)
+   if (!stObj || !stObj->pt)
       return GL_FALSE;
 
    format = stObj->pt->format;