ffmpeg unconnected filter outputs can cause OOM and segfault

[Marton Balint]

Summary of the bug:

Since af1761f7b5b1b72197dc40934953b775c2d951cc ffmpeg no longer detects an unconnected filter output in some cases. This causes frames to queue up and eventually cause an OOM and a segmentation fault.

How to reproduce:

ffmpeg -f lavfi -i testsrc -f lavfi -i testsrc -filter_complex "[0:v][1:v]psnr[out]" -f null none

ffmpeg version N-85544-g9e86a61 Copyright (c) 2000-2017 the FFmpeg developers
  built with gcc 4.8 (SUSE Linux)
  configuration: --disable-vaapi --enable-vdpau --enable-gpl --enable-libfreetype --enable-libzvbi --enable-libass --enable-frei0r --enable-openssl --enable-nonfree --enable-libx264 --enable-libfdk-aac --enable-nonfree --enable-libvorbis --enable-decklink --enable-libmp3lame --enable-openal --enable-libiec61883
  libavutil      55. 61.100 / 55. 61.100
  libavcodec     57. 92.100 / 57. 92.100
  libavformat    57. 72.101 / 57. 72.101
  libavdevice    57.  7.100 / 57.  7.100
  libavfilter     6. 84.101 /  6. 84.101
  libswscale      4.  7.101 /  4.  7.101
  libswresample   2.  8.100 /  2.  8.100
  libpostproc    54.  6.100 / 54.  6.100
Input #0, lavfi, from 'testsrc':
  Duration: N/A, start: 0.000000, bitrate: N/A
    Stream #0:0: Video: rawvideo (RGB[24] / 0x18424752), rgb24, 320x240 [SAR 1:1 DAR 4:3], 25 tbr, 25 tbn, 25 tbc
Input #1, lavfi, from 'testsrc':
  Duration: N/A, start: 0.000000, bitrate: N/A
    Stream #1:0: Video: rawvideo (RGB[24] / 0x18424752), rgb24, 320x240 [SAR 1:1 DAR 4:3], 25 tbr, 25 tbn, 25 tbc
Stream mapping:
  Stream #0:0 (rawvideo) -> psnr:main (graph 0)
  Stream #1:0 (rawvideo) -> psnr:reference (graph 0)
  Stream #0:0 -> #0:0 (rawvideo (native) -> wrapped_avframe (native))
Press [q] to stop, [?] for help
OST: 0x255a920
Output #0, null, to 'none':
  Metadata:
    encoder         : Lavf57.72.101
    Stream #0:0: Video: wrapped_avframe, rgb24, 320x240 [SAR 1:1 DAR 4:3], q=2-31, 200 kb/s, 25 fps, 25 tbn, 25 tbc
    Metadata:
      encoder         : Lavc57.92.100 wrapped_avframe
testsrc: Cannot allocate memory
Segmentation fault (core dumped)

[Marton Balint]

Fixed in 974ee16d6a71c31d0b5db4f139a40831c2f45776.