A fix attached.

```
==37349==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 153770 byte(s) in 1 object(s) allocated from:
    #0 0x7fa249785330 in __interceptor_malloc
(/usr/lib/x86_64-linux-gnu/libasan.so.5+0xe9330)
    #1 0x7fa245c41e59 in WebPMuxAssemble
/libwebp-0.6.1/src/mux/muxedit.c:628
    #2 0x7fa245c4103e in WebPAnimEncoderAssemble
/libwebp-0.6.1/src/mux/anim_encode.c:1562
    #3 0x7fa2485774f9 in libwebp_anim_encode_frame
libavcodec/libwebpenc_animencoder.c:68
    #4 0x7fa24833a6ac in encode_simple_internal libavcodec/encode.c:214
    #5 0x7fa24833a6ac in encode_simple_receive_packet
libavcodec/encode.c:275
    #6 0x7fa24833a6ac in encode_receive_packet_internal
libavcodec/encode.c:309
    #7 0x7fa24833abf8 in avcodec_send_frame libavcodec/encode.c:387
    #8 0x7fa24833af1c in compat_encode libavcodec/encode.c:439
    #9 0x7fa24833b1a5 in avcodec_encode_video2
(/usr/local/ffmpeg/lib/x86_64-linux-gnu/libavcodec.so.58+0x2bb1a5)
```