gl-renderer: Avoid destroying an egl image if known import failed

As observed on some platforms, importing known DMA buffers can cause failures, leading to an attempt of destroying an EGL image not set. This patch resets the num_images such that loop becomes inert when destroying the DMA buffer, and avoids passing an EGL image to it.

The initial import doesn't have this issue as it sets the num_images in case it succeeds. This also corrects the assumption that num_images were 0 at that point which, but if the initial import succeded it was actually set to 1.

Signed-off-by: Marius Vlad marius.vlad@collabora.com