Sanitize assert usage and refuse to compile with NDEBUG.

There were quite a few places where assert() was used with side effects,
 making operation with NDEBUG non-functional.  This commit fixes all the
 cases I know about, but also adds an  #error on NDEBUG because the code
 is untested without assertions and may still have vulnerabilities if
 used without assert.
Fix file hash verification error in deps-win32.yml

libpng was being sha256summed twice, and miniupnpc wasn't being checked at all.
Merge pull request #3344 from gmaxwell/assert_hazards

Sanitize assert usage and refuse to compile with NDEBUG.
Merge pull request #3345 from Michagogo/gitian-descriptor-deps-input-typo

Fix file hash verification error in deps-win32.yml