Fix problems detected as dead assignment warning by clang scan-build.

Wrong assignment by shutdown result hides the real error code
of NewRequestHandler() in ssdp_device.c.
Fix return code description of NewRequestHandler().
Handle return code from ithread_create in sample applications.
Remove unused assignments.

(forward ported from commit 39dfad2cba)

upnp/sample/linux/tv_device_main.c

@@ -53,6 +53,9 @@ int main(int argc, char *argv[])
 
 	/* start a command loop thread */
 	code = ithread_create(&cmdloop_thread, NULL, TvDeviceCommandLoop, NULL);
+	if (code !=  0) {
+		return UPNP_E_INTERNAL_ERROR;
+	}
 #ifdef WIN32
 	ithread_join(cmdloop_thread, NULL);
 #else