libsubcmd: Avoid SEGV/use-after-free when commands aren't excluded

The array shortening may perform unnecessary array copies.

Before commit 657a3ef ("lib subcmd: Avoid memory leak in
exclude_cmds") this was benign, but afterwards this could lead to a
SEGV.

Fixes: 657a3ef ("lib subcmd: Avoid memory leak in exclude_cmds")
Signed-off-by: Ian Rogers <irogers@google.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Chenyuan Mi <cymi20@fudan.edu.cn>
Cc: Ian Rogers <irogers@google.com>
Link: https://lore.kernel.org/r/20230707230926.841086-1-irogers@google.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

Author: captain5050

tools/lib/subcmd/help.c

@@ -68,19 +68,25 @@ void exclude_cmds(struct cmdnames *cmds, struct cmdnames *excludes)
 	while (ci < cmds->cnt && ei < excludes->cnt) {
 		cmp = strcmp(cmds->names[ci]->name, excludes->names[ei]->name);
 		if (cmp < 0) {
-			zfree(&cmds->names[cj]);
-			cmds->names[cj++] = cmds->names[ci++];
+			if (ci == cj) {
+				ci++;
+				cj++;
+			} else {
+				zfree(&cmds->names[cj]);
+				cmds->names[cj++] = cmds->names[ci++];
+			}
 		} else if (cmp == 0) {
 			ci++;
 			ei++;
 		} else if (cmp > 0) {
 			ei++;
 		}
 	}
-
-	while (ci < cmds->cnt) {
-		zfree(&cmds->names[cj]);
-		cmds->names[cj++] = cmds->names[ci++];
+	if (ci != cj) {
+		while (ci < cmds->cnt) {
+			zfree(&cmds->names[cj]);
+			cmds->names[cj++] = cmds->names[ci++];
+		}
 	}
 	for (ci = cj; ci < cmds->cnt; ci++)
 		zfree(&cmds->names[ci]);