Remove two inaccurate asserts:

The assert in LedgerHistory.cpp was incorrect since
getLedgerByHash() can return a null object.  The assert has
been seen to fire inappropriately during online_delete.

The assert in LedgerMaster.cpp was simply misguided.  If a
null hash were returned by getLedgerHashForHistory(), then we
should not forward that hash to peers.  The assert only fires
in a debug build, so a release build would forward the request
for a null hash to peers.

src/ripple/app/ledger/LedgerHistory.cpp

@@ -500,7 +500,6 @@ void LedgerHistory::clearLedgerCachePrior (LedgerIndex seq)
     for (LedgerHash it: m_ledgers_by_hash.getKeys())
     {
         auto const ledger = getLedgerByHash (it);
-        assert(ledger);
         if (!ledger || ledger->info().seq < seq)
             m_ledgers_by_hash.del (it, false);
     }

src/ripple/app/ledger/impl/LedgerMaster.cpp

@@ -504,14 +504,13 @@ LedgerMaster::getFetchPack (LedgerIndex missingIndex,
 {
     auto haveHash = getLedgerHashForHistory(
         missingIndex + 1, reason);
-    if (!haveHash)
+    if (!haveHash || haveHash->isZero())
     {
         JLOG (m_journal.error()) <<
             "No hash for fetch pack. Missing Index " <<
             std::to_string(missingIndex);
         return;
     }
-    assert(haveHash->isNonZero());
 
     // Select target Peer based on highest score.  The score is randomized
     // but biased in favor of Peers with low latency.