Use global mempool nullifier count, not local wallet nullifier count

src/txmempool.h

@@ -201,6 +201,7 @@ public:
      * all inputs are in the mapNextTx array). If sanity-checking is turned off,
      * check does nothing.
      */
+    std::map<uint256, const CTransaction*> getNullifiers();
     void check(const CCoinsViewCache *pcoins) const;
     void setSanityCheck(double dFrequency = 1.0) { nCheckFrequency = static_cast<uint32_t>(dFrequency * 4294967295.0); }