If one calls av_opt_set() with an incorrect string to set the value of
an option of type AV_OPT_TYPE_VIDEO_RATE, the given string is used in a
log message via %s. This also happens when the string is actually a
nullpointer in which case using it for %s is forbidden.

This commit changes this by erroring out early in case of a nullpointer.

This also fixes a warning from GCC 9.2:
"‘%s’ directive argument is null [-Wformat-overflow=]"
Reviewed-by: Anton Khirnov <anton@khirnov.net>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>