Remove incorrect STAmount assert:

When computing rates for offers, an STAmount's value can be out of can be out of
range (before canonicalizing). There was an assert that could incorrectly fire
in some cases. This patch removes that assert.

src/ripple/protocol/impl/STAmount.cpp

@@ -227,7 +227,6 @@ STAmount::STAmount (Issue const& issue,
     , mOffset (exponent)
     , mIsNegative (negative)
 {
-    assert(mValue <= std::numeric_limits<std::int64_t>::max());
     canonicalize ();
 }
 

src/test/app/Offer_test.cpp

@@ -4537,6 +4537,23 @@ public:
         BEAST_EXPECT (++it == offers.end());
     }
 
+    void testFalseAssert()
+    {
+        // An assert was falsely triggering when computing rates for offers.
+        // This unit test would trigger that assert (which has been removed).
+        testcase ("false assert");
+        using namespace jtx;
+
+        Env env{*this};
+        auto const alice = Account ("alice");
+        auto const USD = alice["USD"];
+
+        env.fund (XRP(10000), alice);
+        env.close();
+        env (offer (alice, XRP(100000000000), USD(100000000)));
+        pass();
+    }
+
     void testAll(FeatureBitset features)
     {
         testCanceledOffer(features);
@@ -4607,6 +4624,7 @@ public:
         testAll(all         - flowCross - takerDryOffer);
         testAll(all         - flowCross                );
         testAll(all                                    );
+        testFalseAssert();
     }
 };