Fix read from pointer after free

Fix read from pointer after free

Coverity reported a read from pointer after free defect in
src/mesa/drivers/dri/intel/intel_mipmap_tree.c. Bug# 44205
In intel_miptree_all_slices_resolve() function, i = i->next was
executing after freeing i. I have defined a temporary variable
(next) to store the value of i->next before freeing i

Reported-by: Vinson Lee <vlee@vmware.com>
Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Eric Anholt <eric@anholt.net>