assert if possible when undefined behavior occurs

[shintaro-iwasaki]

Argobots defines when undefined behaviors can happen, but this is not very user friendly. If possible, Argobots should warn users when a debug option is true so that user can easily find and fix the cause of undefined behavior. The idea is that, since the behavior is undefined in such a case, that behavior can be an assertion when a debug option is enabled. Undefined-behavior checks are expensive, so they will be removed for a non-debug configuration.

Examples of when undefined behaviors happen:

• Passing NULL
• Double-freeing an Argobots object (e.g., ABT_pool)
• Double-joining a ULT
• Explicitly freeing an automatic scheduler/pool
• Explicitly freeing a pool of the primary execution stream.

Note that this detection is neither mandatory nor strict since it causes "undefined behavior". Argobots should do the best to show a useful error message rather than incur an error somewhere else, but the user should not expect assertion.

See this thread for related discussion: https://lists.argobots.org/pipermail/discuss/2021-March/000103.html