preferences_backup overwritten when preferences gets damaged

[anonymous]

My computer crashed and as a result, preferences file got truncated to zero size (XFS filesystem does this to files that got damaged). As a result, once I started JOSM again after the computer crashed, josm loaded, the preferences_backup got immediately overwritten by default config (as the file was empty), therefore backup preferences got lost.
I think the preferences_backup in current implementation is pointless (I lost my preferences and I had to restore the file from backup), as it seems to be written on every preferences change.
To server any useful purpose, the preferences should be copied to preferences_backup only on successful, non-crash exit (so if you notice your preferences are lost after starting JOSM, you§ll have still your preferences in preferences%backup, that could still be saved aside and then used to restore your old preferences)

[simon04]

In 9311/josm:

fix #6044 - Do not overwrite preferences_backup when loading preferences fails