Fixup: Style violation
Fix: SEGFAULT in tests_avl_insert_multiple_destroy

The use of `malloc()` caused to allocate an avl structure with
initialized `root` pointer member. At some point when iterating through
this tree of non-existing memory, it segfauls. Allocating the avl
structure with a NULL root node causes the code to allocate _real_
nodes, which _can_ be accessed.
Fix: Possible segfaults, allocate the avl structure with NULLed memory
Remove: test_avl_alloc: We do not test the calloc() function, it has to work