Bugfix: prioritisetransaction: Do some basic sanity checking on txid

Besides giving a nicer error, this also prevents logging arbitrary data (which could have been used to exploit log readers) into debug.log

src/rpcmining.cpp

@@ -288,8 +288,7 @@ Value prioritisetransaction(const Array& params, bool fHelp)
             + HelpExampleRpc("prioritisetransaction", "\"txid\", 0.0, 10000")
         );
 
-    uint256 hash;
-    hash.SetHex(params[0].get_str());
+    uint256 hash = ParseHashStr(params[0].get_str(), "txid");
 
     CAmount nAmount = params[2].get_int64();