Fix memory leak incurred by av1_alloc_txb_buf()

Since alloc_compressor_data() may be called multiple times before
dealloc_compressor_data() is called

The av1_alloc_txb_buf() called by alloc_compressor_data() should
free the txb_buf before allocating new txb_buf

Change-Id: I0fe15de771e0669ec3521e5aac322f5cedfd67a9

av1/encoder/encodetxb.c

@@ -42,6 +42,7 @@ void av1_alloc_txb_buf(AV1_COMP *cpi) {
   int size = ((cm->mi_rows >> MAX_MIB_SIZE_LOG2) + 1) *
              ((cm->mi_cols >> MAX_MIB_SIZE_LOG2) + 1);
 
+  av1_free_txb_buf(cpi);
   // TODO(jingning): This should be further reduced.
   CHECK_MEM_ERROR(cm, cpi->coeff_buffer_base,
                   aom_malloc(sizeof(*cpi->coeff_buffer_base) * size));