fix segmentfault when exec dtagnames or prefcnt without any args

[xuchunmei000]

while exec dtagnames or prefnct without any args, will get segement fault result.

# prefcnt
Segmentation fault (core dumped)

gdb info

Program terminated with signal SIGSEGV, Segmentation fault.
#0  cus__load_btf (cus=0x55eb480132a0, conf=0x0, filename=0x7fddd3215241 "/sys/kernel/btf/vmlinux")