Fix memory corruption in tree index during recovery from snapsht.

When tree.max_size is 0, tree.max_size * 2 is also 0.
Merge branch 'stable'

Conflicts:
	src/box/tree_index.cc
Fix a memory corruption when loading from snapshot.

Implementation of TreeIndex::reserve() had a bug when
tree.members weren't allocated while buildNext was already
using them.

Remove sptree_index_node magic and simplify sptree iterators.

                            </kostja>