use of assert() : move to always-enabled DIAG_ASSERT ?

[fenugrec]

Follow-up to discussion in PR #71 , as well as issue #68 .

There's already some use of assert() throughout the codebase, but there could be more - for instance more argument validation (such as calling diag_l0_send with len == 0), etc.

To have those always-enabled (regardless of NDEBUG), it could be a better idea to use a new macro or wrapper func with our own assert implementation.