bpftool: Fix undefined behavior in qsort(NULL, 0, ...)

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

bpftool: Fix undefined behavior in qsort(NULL, 0, ...)

When netfilter has no entry to display, qsort is called with

qsort(NULL, 0, ...). This results in undefined behavior, as UBSan

reports:

net.c:827:2: runtime error: null pointer passed as argument 1, which is declared to never be null

Although the C standard does not explicitly state whether calling qsort

with a NULL pointer when the size is 0 constitutes undefined behavior,

Section 7.1.4 of the C standard (Use of library functions) mentions:

"Each of the following statements applies unless explicitly stated

otherwise in the detailed descriptions that follow: If an argument to a

function has an invalid value (such as a value outside the domain of

the function, or a pointer outside the address space of the program, or

a null pointer, or a pointer to non-modifiable storage when the

corresponding parameter is not const-qualified) or a type (after

promotion) not expected by a function with variable number of

arguments, the behavior is undefined."

To avoid this, add an early return when nf_link_info is NULL to prevent

calling qsort with a NULL pointer.