The argument pertaining to a printf %s conversion specifier must not
be NULL, even if the precision (i.e. the number of characters to write)
is zero. If it is NULL, it is undefined behaviour.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
(cherry picked from commit 6de6ce7bc80e874099895b6c73977bc2efb06a4d)