Improve canonicalization of serialized amounts:

The existing code that deserialized an STAmount was sub-optimal and performed
poorly. In some rare cases the operation could result in otherwise valid
serialized amounts overflowing during deserialization. This commit will help
detect error conditions more quickly and eliminate the problematic corner cases.

src/ripple/app/misc/impl/TxQ.cpp

@@ -290,6 +290,8 @@ TxQ::MaybeTx::apply(Application& app, OpenView& view, beast::Journal j)
 {
     // If the rules or flags change, preflight again
     assert(pfresult);
+    STAmountSO stAmountSO{view.rules().enabled(fixSTAmountCanonicalize)};
+
     if (pfresult->rules != view.rules() || pfresult->flags != flags)
     {
         JLOG(j.debug()) << "Queued transaction " << txID
@@ -728,6 +730,8 @@ TxQ::apply(
     ApplyFlags flags,
     beast::Journal j)
 {
+    STAmountSO stAmountSO{view.rules().enabled(fixSTAmountCanonicalize)};
+
     // See if the transaction paid a high enough fee that it can go straight
     // into the ledger.
     if (auto directApplied = tryDirectApply(app, view, tx, flags, j))