6091 avl_add doesn't assert on non-debug builds

illumos/illumos-gate@faa2b6be2fc102adf9ed584fc1a667b4ddf50d78

https://www.illumos.org/issues/6091

Long story short, avl_add's use of ASSERT(0) can cause really strange looking
crashes on non-debug builds of libavl.so because ASSERTs turn into no-ops.
...

Reviewed by: Andy Stormont <astormont@racktopsystems.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Steve Dougherty <steve@asksteved.com>
Approved by: Dan McDonald <danmcd@omniti.com>
Author: Josef 'Jeff' Sipek <josef.sipek@nexenta.com>