Blender Git Loki

Fix T51913: Context tab for textures issue

The original code was doing a sanity check to see if existing index was
out of range. However the comparison was wrong.

So if the previous ct->user (active index of texture node) was larger
than then number of available texture nodes + 1 in the other material,
we would never re-set the index to 0.

Bug introduced on c31f74de6bb7.

There was an early attempt of fixing this (2b2ac5d3cc) but it was just working
by pure, luck. And failing in cases like the one from this bug report.