Version of OpenTTD

master

Expected result

Config file is correct (zero length string value) when a SDTG_MMANY/ManyOfMany setting has no bits set.

Actual result

Uninitialised bytes may be written into the config file when a SDTG_MMANY/ManyOfMany setting has no bits set.
Clearing all bits of display_opt may result in the config file line for display_opt having uninitialised bytes written for the value.
This may cause an error at config load time, and/or make the config file invalid UTF-8, etc.

ManyOfManySettingDesc::FormatValue does not write any bytes if there are no bits set, and the output buffer is not initialised, and therefore not zero-terminated after FormatValue is called.
(StringSettingDesc::FormatValue has extra code to handle this special case).

See also: JGRennison/OpenTTD-patches#344

Steps to reproduce

1. Clear all bits of display_opt in the usual way.
2. Exit OpenTTD to write config.
3. Open OpenTTD and exit to write config again if necessary.