perf subcmd: avoid crash in exclude_cmds when excludes is empty

When cross-compiling the perf tool for ARM64, `perf help` may crash
with the following assertion failure:

  help.c:122: exclude_cmds: Assertion `cmds->names[ci] == NULL' failed.

This happens when the perf binary is not named exactly "perf" or when
multiple "perf-*" binaries exist in the same directory. In such cases,
the `excludes` command list can be empty, which leads to the final
assertion in exclude_cmds() being triggered.

Add a simple guard at the beginning of exclude_cmds() to return early
if excludes->cnt is zero, preventing the crash.

Signed-off-by: hupu <hupu.gm@gmail.com>
Reported-by: Guilherme Amadio <amadio@gentoo.org>
Reviewed-by: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/r/20250909094953.106706-1-amadio@gentoo.org
Signed-off-by: Namhyung Kim <namhyung@kernel.org>

tools/lib/subcmd/help.c

@@ -75,6 +75,9 @@ void exclude_cmds(struct cmdnames *cmds, struct cmdnames *excludes)
 	size_t ci, cj, ei;
 	int cmp;
 
+	if (!excludes->cnt)
+		return;
+
 	ci = cj = ei = 0;
 	while (ci < cmds->cnt && ei < excludes->cnt) {
 		cmp = strcmp(cmds->names[ci]->name, excludes->names[ei]->name);