ffplay: skip SHOW_MODE_VIDEO while cycling show modes with "w" key

[ami_stuff]

I think it would be good to skip "SHOW_MODE_VIDEO" while cycling modes with "w" key when input have no video stream - maybe the same for video without audio (skip SHOW_MODE_WAVES and SHOW_MODE_RDFT), so in result no cycle, since there is nothing to cycle - only black screens.

C:\>ffplay g723_1.wav
ffplay version N-49268-ge9d443c Copyright (c) 2003-2013 the FFmpeg developers
  built on Jan 24 2013 23:27:01 with gcc 4.7.2 (GCC)
  configuration: --enable-gpl --enable-version3 --disable-w32threads --enable-av
isynth --enable-bzlib --enable-fontconfig --enable-frei0r --enable-gnutls --enab
le-libass --enable-libbluray --enable-libcaca --enable-libfreetype --enable-libg
sm --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --e
nable-libopenjpeg --enable-libopus --enable-librtmp --enable-libschroedinger --e
nable-libspeex --enable-libtheora --enable-libtwolame --enable-libvo-aacenc --en
able-libvo-amrwbenc --enable-libvorbis --enable-libvpx --enable-libx264 --enable
-libxavs --enable-libxvid --enable-zlib
  libavutil      52. 15.103 / 52. 15.103
  libavcodec     54. 90.100 / 54. 90.100
  libavformat    54. 61.104 / 54. 61.104
  libavdevice    54.  3.102 / 54.  3.102
  libavfilter     3. 33.100 /  3. 33.100
  libswscale      2.  2.100 /  2.  2.100
  libswresample   0. 17.102 /  0. 17.102
  libpostproc    52.  2.100 / 52.  2.100
[wav @ 02658880] max_analyze_duration 5000000 reached at 5100000 microseconds
[wav @ 02658880] Estimating duration from bitrate, this may be inaccurate
Input #0, wav, from 'g723_1.wav':
  Duration: 00:00:30.07, bitrate: 6 kb/s
    Stream #0:0: Audio: g723_1 (B[0][0][0] / 0x0042), 8000 Hz, mono, s16, 6 kb/s

   5.81 A-V:  0.000 fd=   0 aq=   15KB vq=    0KB sq=    0B f=0/0

[Marton Balint]

Fixed in latest git master.