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/core_read.cpp

@@ -131,6 +131,11 @@ uint256 ParseHashUV(const UniValue& v, const string& strName)
     string strHex;
     if (v.isStr())
         strHex = v.getValStr();
+    return ParseHashStr(strHex, strName);  // Note: ParseHashStr("") throws a runtime_error
+}
+
+uint256 ParseHashStr(const std::string& strHex, const std::string& strName)
+{
     if (!IsHex(strHex)) // Note: IsHex("") is false
         throw runtime_error(strName+" must be hexadecimal string (not '"+strHex+"')");