test_firmware: fix setting old custom fw path back on exit, second try

Commit 65c79230576 tried to clear the custom firmware path on exit by
writing a single space to the firmware_class.path parameter.  This
doesn't work because nothing strips this space from the value stored
and fw_get_filesystem_firmware() only ignores zero-length paths.

Instead, write a null byte.

Fixes: 0a8adf58475 ("test: add firmware_class loader test")
Fixes: 65c79230576 ("test_firmware: fix setting old custom fw path back on exit")
Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Acked-by: Luis R. Rodriguez <mcgrof@kernel.org>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

tools/testing/selftests/firmware/fw_lib.sh

@@ -154,11 +154,13 @@ test_finish()
 	if [ "$HAS_FW_LOADER_USER_HELPER" = "yes" ]; then
 		echo "$OLD_TIMEOUT" >/sys/class/firmware/timeout
 	fi
-	if [ "$OLD_FWPATH" = "" ]; then
-		OLD_FWPATH=" "
-	fi
 	if [ "$TEST_REQS_FW_SET_CUSTOM_PATH" = "yes" ]; then
-		echo -n "$OLD_FWPATH" >/sys/module/firmware_class/parameters/path
+		if [ "$OLD_FWPATH" = "" ]; then
+			# A zero-length write won't work; write a null byte
+			printf '\000' >/sys/module/firmware_class/parameters/path
+		else
+			echo -n "$OLD_FWPATH" >/sys/module/firmware_class/parameters/path
+		fi
 	fi
 	if [ -f $FW ]; then
 		rm -f "$FW"