SyntheticClient.cc: check return value of getdir()

SyntheticClient.cc: check return value of getdir()

Check return value as done in all other places. Fix error messages
to print correct function name getdir and not read_dir/readdir since
the error isn't necessarily raised by read_dir().

Fix for:
CID 1219463 (#1 of 1): Unchecked return value (CHECKED_RETURN)
 check_return: Calling getdir without checking return value (as
 is done elsewhere 4 out of 5 times).

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>