bpftool: Check malloc return value in mount_bpffs_for_pin

Fix and add a missing NULL check for the prior malloc() call.

Fixes: 49a086c2 ("bpftool: implement prog load command")
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Quentin Monnet <quentin@isovalent.com>
Acked-by: Roman Gushchin <guro@fb.com>
Link: https://lore.kernel.org/bpf/20210715110609.29364-1-tklauser@distanz.ch