opus_decode fails on >20ms zero-length frame following hybrid frame

opus_decode fails with OPUS_BAD_ARG on a packet containing a 40ms or 60ms LP zero-length frame when it follows a hybrid or MDCT frame.

The attached ogg opus file contains ten 40ms packets. The duration of the expected output is 400ms - 6.5ms pre-skip = 393.5ms. However, opus_decode and the opusdec tool fail after the first packet. The 1.0 reference implementation decodes it successfully.

The first packet contains two 20ms Hybrid SWB frames, and the others each contain one 40ms LP WB frame. The first 40ms LP WB frame has a length of 0, invoking PLC.