Tidy up SHAMap node handling of invalid wire formats

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

@@ -239,6 +239,9 @@ public:
                     Blob (node.nodedata().begin(), node.nodedata().end()),
                     0, snfWIRE, uZero, false);
 
+                if (!newNode)
+                    return;
+
                 s.erase();
                 newNode->addRaw(s, snfPREFIX);