libbpf: Fix potential NULL pointer dereference

Wait until after the UMEM is checked for null to dereference it.

Fixes: 43f1bc1e

 ("libbpf: Restore umem state after socket create failure")
Signed-off-by: Ciara Loftus <ciara.loftus@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20210408052009.7844-1-ciara.loftus@intel.com