client/Client.cc: fix possible NULL pointer dereference

CID 751332 Dereference null return value (CWE-476)

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>
client/SyntheticClient.cc: check return value of describe_layout()

Fix SyntheticClient::chunk_file(): check return value of
describe_layout() and handle the error.

CID 966615 Unchecked return value (CWE-252)

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