Don't initialize TLI variable to -1, as TimeLineID is unsigned.

This was causing a compiler warning with Solaris compiler. Use 0 instead.
The variable is initialized just for the sake of tidyness  and/or debugging,
it's not used for anything before setting it to a real value.

Per report and suggestion from Peter Eisentraut.

src/backend/replication/walreceiver.c

@@ -73,7 +73,7 @@ walrcv_disconnect_type walrcv_disconnect = NULL;
  * corresponding the filename of recvFile, used for error messages.
  */
 static int	recvFile = -1;
-static TimeLineID	recvFileTLI = -1;
+static TimeLineID	recvFileTLI = 0;
 static XLogSegNo recvSegNo = 0;
 static uint32 recvOff = 0;