When introducing the fmt_day_heading() function in commit d56cc7a (Make
heading in appointments panel configurable, 2017-08-19), we forgot to
initialize the dynamic string. This resulted in calling mem_free() with
a garbage pointer, potentially resulting in a segmentation fault. Fix
this by calling string_init() before using the string.
Signed-off-by: Lars Henriksen <LarsHenriksen@get2net.dk>
Signed-off-by: Lukas Fleischer <lfleischer@calcurse.org>