selftests/bpf: Prevent infinite loop in veristat when base file is too short
The following example forces veristat to loop indefinitely:
$ cat two-ok
file_name,prog_name,verdict,total_states
file-a,a,success,12
file-b,b,success,67
$ cat add-failure
file_name,prog_name,verdict,total_states
file-a,a,success,12
file-b,b,success,67
file-b,c,failure,32
$ veristat -C two-ok add-failure
<does not return>
The loop is caused by handle_comparison_mode() not checking if `base`
variable points to `fallback_stats` prior advancing joined results
using `base`.
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20230407154125.896927-1-eddyz87@gmail.com