analyzer/protocol/dns: fix memory leak

If `dns_TSIG_addl` is not set, then the BroString allocated by
ExtractOctets() leaks.  Therefore, don't ask ExtractOctets() to copy
the data to a BroString if we're not going to use it.

Yet another memory leak (out of way too many) which would have been
prevented by using smart pointers.

src/analyzer/protocol/dns/DNS.cc

@@ -769,7 +769,7 @@ int DNS_Interpreter::ParseRR_TSIG(DNS_MsgInfo* msg,
 	unsigned int sign_time_msec = ExtractShort(data, len);
 	unsigned int fudge = ExtractShort(data, len);
 	BroString* request_MAC;
-	ExtractOctets(data, len, &request_MAC);
+	ExtractOctets(data, len, dns_TSIG_addl ? &request_MAC : nullptr);
 	unsigned int orig_id = ExtractShort(data, len);
 	unsigned int rr_error = ExtractShort(data, len);
 	ExtractOctets(data, len, 0);  // Other Data