ws_pipe: don't check for negative number using DWORD.

DWORD on windows is unsigned, then there is no point in checking
for negative values.

Change-Id: I0b03fb19ebdff86e610cd4571fc30c49b7bd1284
Reviewed-on: https://code.wireshark.org/review/27766
Petri-Dish: Dario Lombardo <lomato@gmail.com>
Tested-by: Petri Dish Buildbot
Reviewed-by: Anders Broman <a.broman58@gmail.com>

wsutil/ws_pipe.c

@@ -484,7 +484,7 @@ ws_read_string_from_pipe(ws_pipe_handle read_pipe, gchar *buffer,
         {
             break;
         }
-        if (bytes_avail <= 0)
+        if (bytes_avail == 0)
         {
             ret = TRUE;
             break;