bpftool: Fix NULL pointer dereference when pin {PROG, MAP, LINK} without FILE

bpftool: Fix NULL pointer dereference when pin {PROG, MAP, LINK} without FILE

[ Upstream commit 34de8e6e0e1f66e431abf4123934a2581cb5f133 ]

When using bpftool to pin {PROG, MAP, LINK} without FILE,
segmentation fault will occur. The reson is that the lack
of FILE will cause strlen to trigger NULL pointer dereference.
The corresponding stacktrace is shown below:

do_pin
  do_pin_any
    do_pin_fd
      mount_bpffs_for_pin
        strlen(name) <- NULL pointer dereference

Fix it by adding validation to the common process.

Fixes: 75a1e792c335 ("tools: bpftool: Allow all prog/map handles for pinning objects")
Signed-off-by: Pu Lehui <pulehui@huawei.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Quentin Monnet <quentin@isovalent.com>
Link: https://lore.kernel.org/bpf/20221102084034.3342995-1-pulehui@huaweicloud.com
Signed-off-by: Sasha Levin <sashal@kernel.org>