Blackfin: strncpy: fix handling of zero lengths

The jump to 4f will cause the NUL padding loop to run at least one time,
so if string length is zero just jump to the end.  Otherwise we wrongly
write one NUL byte when size==0.

Signed-off-by: Steven Miao <realmz6@gmail.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>