pid1: fix (harmless) off-by-one in PATH_MAX comparison

PATH_MAX is supposed to include the terminating NUL byte. But we already
check that there is no NUL byte in the specified path. Hence the maximum
length we can expect is PATH_MAX - 1.

This doesn't change much, but makes this use of PATH_MAX consistent with the
rest of the codebase.

src/core/manager.c

@@ -2222,7 +2222,7 @@ static unsigned manager_dispatch_dbus_queue(Manager *m) {
 
 static int manager_dispatch_cgroups_agent_fd(sd_event_source *source, int fd, uint32_t revents, void *userdata) {
         Manager *m = userdata;
-        char buf[PATH_MAX+1];
+        char buf[PATH_MAX];
         ssize_t n;
 
         n = recv(fd, buf, sizeof(buf), 0);