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

In the Linux kernel, the following vulnerability has been resolved:

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

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.