Fix null pointer crash in audio preferences.

When MuseScore is set to use Jack, the PortAudio driver returns null.
Fix null pointer crash in preferences

When turning off "use JackAudio" in preferences, l and r get set to null.
In this case, skip the section that uses them.
Merge pull request #207 from softins/preferences-crash-fix

Preferences crash fix
Accept '+' as both a prefix and a suffix, to accommodate harmony notation.

Also, added '0' (with usual suffix combination) to font and to acceptable digits.
Merge pull request #209 from mgavioli/19981_figured_bass_cross_as_prefix

Figured Bass: Accept '+' as both a prefix and a suffix
small low level optimizations
merge with head
add first tuplet regression test
update for workspaces
fix build