Fixed leaks in LM::operator[](const char *) and LM::clear_oov()

Fixed leaks in LM::operator[](const char *) and LM::clear_oov()

LM::clear_oov() (also known as clear_rest) resizes LM::oov without
release memory allocated for its strings

LM::operator[](const char *) abused LM::lm->HT to index LM::oov.
This has a nasty effect that lm->HT keeps growing no matter you call
LM::clear_oov(). With lm->HT's growing bigger and bigger, hash lookup
slows down significantly.

The new implementation uses another hash for index oov and free it
when LM::clear_oov() is called