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

tools/testing/selftests/bpf/veristat.c

@@ -1824,18 +1824,22 @@ static int handle_comparison_mode(void)
 			join->stats_b = comp;
 			i++;
 			j++;
-		} else if (comp == &fallback_stats || r < 0) {
+		} else if (base != &fallback_stats && (comp == &fallback_stats || r < 0)) {
 			join->file_name = base->file_name;
 			join->prog_name = base->prog_name;
 			join->stats_a = base;
 			join->stats_b = NULL;
 			i++;
-		} else {
+		} else if (comp != &fallback_stats && (base == &fallback_stats || r > 0)) {
 			join->file_name = comp->file_name;
 			join->prog_name = comp->prog_name;
 			join->stats_a = NULL;
 			join->stats_b = comp;
 			j++;
+		} else {
+			fprintf(stderr, "%s:%d: should never reach here i=%i, j=%i",
+				__FILE__, __LINE__, i, j);
+			return -EINVAL;
 		}
 		env.join_stat_cnt += 1;
 	}