Fix a possible crash

If the dictionary ID, as indicated in the metadata, is
less than three chars long, it crashes LatinIME.
Of course we don't have such dictionary IDs in the current
metadata, but it's still better to be able to handle
the case gracefully ^^;

Change-Id: I60cdf6f8ecce9f4d44b42ddd5d157aebff9a4163