Unmapped lavfi outputs with unlabelled pads automatically included

[Gyan]

Summary of the bug:

Filtergraph outputs without a labelled pad are automatically included in the output even with -map. If labelled, expected "unconnected output" error is shown.

How to reproduce:

ffmpeg -f lavfi -i testsrc -f lavfi -i testsrc -filter_complex "[0:v][1:v]psnr[out];[0]null" -map [out] -f null -

ffmpeg version N-90168-gc0919c4985 Copyright (c) 2000-2018 the FFmpeg developers
  built with gcc 7.3.0 (Rev1, Built by MSYS2 project)
  configuration:  --enable-libmp3lame --enable-libopus --enable-libvorbis --enable-libvpx --enable-libx264 --enable-libx265 --enable-fontconfig --enable-libass --enable-libbluray --enable-libfreetype --enable-libmfx --enable-libmysofa --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-libsnappy --enable-libsoxr --enable-libspeex --enable-libtheora --enable-libtwolame --enable-libvidstab --enable-libvo-amrwbenc --enable-libwavpack --enable-libwebp --enable-libxml2 --enable-libzimg --enable-libshine --enable-gpl --enable-openssl --enable-avisynth --enable-chromaprint --enable-decklink --enable-frei0r --enable-libbs2b --enable-libcaca --enable-libcdio --enable-libfdk-aac --enable-libflite --enable-libfribidi --enable-libgme --enable-libgsm --enable-libilbc --enable-libkvazaar --enable-libmodplug --enable-libopenh264 --enable-libopenmpt --enable-librtmp --enable-librubberband --enable-libssh --enable-libtesseract --enable-libxavs --enable-libxvid --enable-libzmq --enable-libzvbi --enable-opencl --enable-opengl --enable-libcodec2 --extra-cflags=-fopenmp --extra-libs=-lgomp --extra-cflags=-DLIBTWOLAME_STATIC --extra-libs=-lstdc++ --extra-cflags=-DLIBSSH_STATIC --extra-ldflags='-Wl,--allow-multiple-definition' --extra-cflags=-DCACA_STATIC --extra-cflags=-DMODPLUG_STATIC --extra-cflags=-DCHROMAPRINT_NODLL --extra-libs=-lstdc++ --extra-cflags=-DZMQ_STATIC --extra-libs=-lpsapi --extra-cflags=-DLIBXML_STATIC --disable-w32threads --extra-cflags=-DKVZ_STATIC_LIB --enable-version3 --enable-nonfree --disable-debug
  libavutil      56.  7.101 / 56.  7.101
  libavcodec     58. 13.100 / 58. 13.100
  libavformat    58. 10.100 / 58. 10.100
  libavdevice    58.  2.100 / 58.  2.100
  libavfilter     7. 12.100 /  7. 12.100
  libswscale      5.  0.101 /  5.  0.101
  libswresample   3.  0.101 /  3.  0.101
  libpostproc    55.  0.100 / 55.  0.100
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
  Stream #0:0 (rawvideo) -> null
  Stream #1:0 (rawvideo) -> psnr:reference
  null -> Stream #0:0 (wrapped_avframe)
  psnr -> Stream #0:1 (wrapped_avframe)
Press [q] to stop, [?] for help
Output #0, null, to 'pipe:':
  Metadata:
    encoder         : Lavf58.10.100
    Stream #0:0: Video: wrapped_avframe, rgb24(progressive), 320x240 [SAR 1:1 DAR 4:3], q=2-31, 200 kb/s, 25 fps, 25 tbn, 25 tbc
    Metadata:
      encoder         : Lavc58.13.100 wrapped_avframe
    Stream #0:1: Video: wrapped_avframe, gbrp, 320x240 [SAR 1:1 DAR 4:3], q=2-31, 200 kb/s, 25 fps, 25 tbn, 25 tbc
    Metadata:
      encoder         : Lavc58.13.100 wrapped_avframe
frame=   25 fps=0.0 q=-0.0 Lq=-0.0 size=N/A time=00:00:01.00 bitrate=N/A speed=50.8x
video:26kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: unknown
[Parsed_psnr_0 @ 0000000002a8b000] PSNR r:inf g:inf b:inf average:inf min:inf max:inf