bus/dir-watch:Do not crash when reloading configuration with > 128 dirs

When dbus-daemon sets more than 128 directories for XDG_DATA_DIRS, during the first call to _set_watched_dirs_internal, none of the elements in new_dirs will be NULL. If the memory after new_dirs is also not NULL, then num_wds will be greater than 128. After sending SIGHUP to dbus-daemon, dbus-daemon calls _set_watched_dirs_internal for the second time. At this time, j in if (dirs[j] && strcmp (new_dirs[i], dirs[j]) == 0) may be greater than 128, which could cause strcmp to trigger a coredump.

We can limit the size of the array index to prevent this type of coredump from occurring

Fix: #481 (closed)