lib: silence null-dereference warnings

In debug mode, MingGW-w64's GCC 7.3 issues null-dereference warnings
when dereferencing pointers after DEBUGASSERT-ing that they are not
NULL.
Fix this by removing the DEBUGASSERTs.

Suggested-by: Daniel Stenberg
Ref: https://github.com/curl/curl/pull/2463