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.

src/wallet.cpp

@@ -1298,7 +1298,9 @@ bool CWallet::CreateTransaction(const vector<pair<CScript, int64_t> >& vecSend,
 
                         // Reserve a new key pair from key pool
                         CPubKey vchPubKey;
-                        assert(reservekey.GetReservedKey(vchPubKey)); // should never fail, as we just unlocked
+                        bool ret;
+                        ret = reservekey.GetReservedKey(vchPubKey);
+                        assert(ret); // should never fail, as we just unlocked
 
                         scriptChange.SetDestination(vchPubKey.GetID());
                     }