Don't double-free mat.texture

[MichaelChirico]

mat.texture = NULL itself does mat.texture->unref():

rgl/src/types.h

Line 63 in 8b32d9c

Ref& operator = (T* in_ptr) { if (ptr) ptr->unref(); ptr = in_ptr; if (ptr) ptr->ref(); return *this; }

So this is actually a double-free.

This was flagged by ThreadSanitizer on clang.