GUACAMOLE-377: Correct return value confusion in handling of timeout …

…vs. general failure of WaitForMultipleObjects().

The conversion of WAIT_FAILED to a signed int and back may cause the
value of result to not actually match the value of the WAIT_FAILED macro
due to the difference in size and sign extension during conversion.

src/protocols/rdp/rdp.c

@@ -407,7 +407,7 @@ static int rdp_guac_client_wait_for_messages(guac_client* client,
             GUAC_RDP_MAX_FILE_DESCRIPTORS);
 
     /* Wait for data and construct a reasonable frame */
-    int result = WaitForMultipleObjects(num_handles, handles, FALSE,
+    DWORD result = WaitForMultipleObjects(num_handles, handles, FALSE,
             timeout_msecs);
 
     /* Translate WaitForMultipleObjects() return values */