It seems vocabulary size of data_clas and data_lm are different. I guess the problem is caused by different preprocessing used in data_clas and data_lm. To check my guess I simply used
data_clas.vocab.itos = data_lm.vocab.itos
Before the following line
learn_c = text_classifier_learner(data_clas, AWD_LSTM, drop_mult=0.3)
This has fixed the error.