Fix: overwrite leftover ffmpeg temp file if a prior conversion failed

[gilbertgong]

In our long running whisper.cpp instance, we ran into an issue where ffmpeg failed to convert a single file, and because it failed - it left the resultant target temp file. Subsequent requests failed to process because ffmpeg declined to overwrite the existing target file:

ffmpeg version 4.4.2-0ubuntu0.22.04.1 Copyright (c) 2000-2021 the FFmpeg developers
  built with gcc 11 (Ubuntu 11.2.0-19ubuntu1)
  configuration: --prefix=/usr --extra-version=0ubuntu0.22.04.1 --toolchain=hardened --libdir=/usr/lib/x86_64-linux-gnu --incdir=/usr/include/x86_64-linux-gnu --arch=amd64 --enable-gpl --disable-stripping --enable-gnutls --enable-ladspa --enable-libaom --enable-libass --enable-libbluray --enable-libbs2b --enable-libcaca --enable-libcdio --enable-libcodec2 --enable-libdav1d --enable-libflite --enable-libfontconfig --enable-libfreetype --enable-libfribidi --enable-libgme --enable-libgsm --enable-libjack --enable-libmp3lame --enable-libmysofa --enable-libopenjpeg --enable-libopenmpt --enable-libopus --enable-libpulse --enable-librabbitmq --enable-librubberband --enable-libshine --enable-libsnappy --enable-libsoxr --enable-libspeex --enable-libsrt --enable-libssh --enable-libtheora --enable-libtwolame --enable-libvidstab --enable-libvorbis --enable-libvpx --enable-libwebp --enable-libx265 --enable-libxml2 --enable-libxvid --enable-libzimg --enable-libzmq --enable-libzvbi --enable-lv2 --enable-omx --enable-openal --enable-opencl --enable-opengl --enable-sdl2 --enable-pocketsphinx --enable-librsvg --enable-libmfx --enable-libdc1394 --enable-libdrm --enable-libiec61883 --enable-chromaprint --enable-frei0r --enable-libx264 --enable-shared
  libavutil      56. 70.100 / 56. 70.100
  libavcodec     58.134.100 / 58.134.100
  libavformat    58. 76.100 / 58. 76.100
  libavdevice    58. 13.100 / 58. 13.100
  libavfilter     7.110.100 /  7.110.100
  libswscale      5.  9.100 /  5.  9.100
  libswresample   3.  9.100 /  3.  9.100
  libpostproc    55.  9.100 / 55.  9.100
[wav @ 0x5950add2a6c0] Estimating duration from bitrate, this may be inaccurate
Guessed Channel Layout for Input Stream #0.0 : mono
Input #0, wav, from 'whisper_server_temp_file.wav':
  Duration: 00:00:03.30, bitrate: 768 kb/s
  Stream #0:0: Audio: pcm_s16le ([1][0][0][0] / 0x0001), 48000 Hz, mono, s16, 768 kb/s
File 'whisper_server_temp_file.wav_temp.wav' already exists. Overwrite? [y/N] Not overwriting - exiting

Deleting the file manually allows it to again proceed. This patch preemptively removes the file just in case it exists to fix this potential issue. adds the -y flag to ffmpeg instructing it to overwrite if necessary.

[jensdraht1999]

Yeah seems to be good idea, but you could after "-i" pipe give a "-y" pipe (which means always say yes).

So this here:
cmd_stream << "ffmpeg -i "" << temp_filename << "" -ar 16000 -ac 1 -c:a pcm_s16le "" << converted_filename_temp << "" 2>&1";

would become this:
cmd_stream << "ffmpeg -i "" << temp_filename << "" -y -ar 16000 -ac 1 -c:a pcm_s16le "" << converted_filename_temp << "" 2>&1";

[gilbertgong]

Yeah seems to be good idea, but you could after "-i" pipe give a "-y" pipe (which means always say yes).

So this here: cmd_stream << "ffmpeg -i "" << temp_filename << "" -ar 16000 -ac 1 -c:a pcm_s16le "" << converted_filename_temp << "" 2>&1";

would become this: cmd_stream << "ffmpeg -i "" << temp_filename << "" -y -ar 16000 -ac 1 -c:a pcm_s16le "" << converted_filename_temp << "" 2>&1";

Sounds good, I've made this change.