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 destroyng 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 the num_images
were 0 at that point which, if the initial import succeded, was actually set
to 1.

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