libsemanage: never call memcpy with a NULL value

clang's static analyzer reports "Argument with 'nonnull' attribute
passed null" in append_str(), because argument t may be NULL but is used
in a call to memcpy().

Make append_str() do nothing when called with t=NULL.

Signed-off-by: Nicolas Iooss <nicolas.iooss@m4x.org>
1 file changed