keys_save_bindings(): Skip unset key bindings
keys_save_bindings(): Skip unset key bindings

Saving unset key bindings currently results in a segmentation fault or
invalid key specifiers being stored. Skip these entries to avoid this.

Signed-off-by: Lukas Fleischer <af90e31281d8305af43dc616c5aaca3c7165f8a7@cryptocrack.de>

lfos/calcurse,lfos/calcurse,lfos/calcurse