Fix missing locking in CTxMemPool::setSanityCheck(double dFrequency)

* writing variable 'nCheckFrequency' requires holding mutex 'cs'

src/txmempool.h

@@ -529,7 +529,7 @@ public:
      * check does nothing.
      */
     void check(const CCoinsViewCache *pcoins) const;
-    void setSanityCheck(double dFrequency = 1.0) { nCheckFrequency = static_cast<uint32_t>(dFrequency * 4294967295.0); }
+    void setSanityCheck(double dFrequency = 1.0) { LOCK(cs); nCheckFrequency = static_cast<uint32_t>(dFrequency * 4294967295.0); }
 
     // addUnchecked must updated state for all ancestors of a given transaction,
     // to track size/count of descendant transactions.  First version of