The issue was caused by usage of non-initialized image user, which
could have different settings, causing some random image being loaded
or not loaded at all.
This caused non-deterministic behavior of Cycles image loading because
it was querying image information from several places.
This fixes crash reported in T50616, but it's not a complete fix
because preview rendering in material is wrong (same wrong as in
2.78a release).
e76364adcd