row_merge_buf_add(): Correctly calculate buf->total_size.

row_merge_buf_write(): Add an assertion about buf->total_size.

row_merge_heap_create(), row_merge_read_rec(), row_merge_insert_index_tuples():
offsets needs to be 1 + REC_OFFS_HEADER_SIZE + n_fields.  Previously, we
allocated one element too little.

row_merge_blocks(): Use goto instead of break in the AT_END argument
of ROW_MERGE_WRITE_GET_NEXT.