"frame size not set" muxing AAC alone to mpegts

[DeHackEd]

Summary of the bug:

Error message when muxing to mpegts containing only AAC audio.

[mpegts @ 0x17754c0] frame size not set

How to reproduce:

% ffmpeg -f lavfi -i anullsrc -c aac -f mpegts testbroken.ts
ffmpeg version N-91396-g27662ed Copyright (c) 2000-2018 the FFmpeg developers
  built with gcc 4.4.7 (GCC) 20120313 (Red Hat 4.4.7-18)
  configuration: --enable-libx264 --extra-cflags='-march=core2' --enable-gpl --enable-libfdk-aac --enable-nonfree --disable-vdpau --disable-outdevs --disable-hwaccels --disable-indevs --disable-lzma --disable-bzlib --enable-zlib --disable-libx265 --enable-indev=lavfi
  libavutil      56. 18.102 / 56. 18.102
  libavcodec     58. 20.104 / 58. 20.104
  libavformat    58. 17.101 / 58. 17.101
  libavdevice    58.  4.101 / 58.  4.101
  libavfilter     7. 25.100 /  7. 25.100
  libswscale      5.  2.100 /  5.  2.100
  libswresample   3.  2.100 /  3.  2.100
  libpostproc    55.  2.100 / 55.  2.100
Input #0, lavfi, from 'anullsrc':
  Duration: N/A, start: 0.000000, bitrate: 705 kb/s
    Stream #0:0: Audio: pcm_u8, 44100 Hz, stereo, u8, 705 kb/s
Stream mapping:
  Stream #0:0 -> #0:0 (pcm_u8 (native) -> aac (native))
Press [q] to stop, [?] for help
[mpegts @ 0x2db9280] frame size not set
Output #0, mpegts, to 'testbroken.ts':
  Metadata:
    encoder         : Lavf58.17.101
    Stream #0:0: Audio: aac (LC), 44100 Hz, stereo, fltp, 128 kb/s
    Metadata:
      encoder         : Lavc58.20.104 aac
size=     634kB time=00:13:05.48 bitrate=   6.6kbits/s speed= 212x    
video:0kB audio:200kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 216.485977%
[aac @ 0x2dba9c0] Qavg: 65536.000

The output doesn't appear broken though.

[Carl Eugen Hoyos]

Regression since 6f69f7a8 / 9200514a