[postprocessor/ffmpeg] Don't let ffmpeg read from stdin (fixes #4945)

If you run 'while read aurl ; do youtube-dl --extract-audio "${aurl}"; done < path_to_batch_file'  (batch_file contains one url per line) each call to youtube-dl consumed some characters and 'read' would assing to 'aurl' a non valid url, something like 'tube.com/watch?v=<id>'.

youtube_dl/postprocessor/ffmpeg.py

@@ -95,7 +95,7 @@ class FFmpegPostProcessor(PostProcessor):
 
         if self._downloader.params.get('verbose', False):
             self._downloader.to_screen('[debug] ffmpeg command line: %s' % shell_quote(cmd))
-        p = subprocess.Popen(cmd, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
+        p = subprocess.Popen(cmd, stdout=subprocess.PIPE, stderr=subprocess.PIPE, stdin=subprocess.PIPE)
         stdout, stderr = p.communicate()
         if p.returncode != 0:
             stderr = stderr.decode('utf-8', 'replace')
@@ -134,7 +134,7 @@ class FFmpegExtractAudioPP(FFmpegPostProcessor):
                 encodeFilename(self._probe_executable, True),
                 encodeArgument('-show_streams'),
                 encodeFilename(self._ffmpeg_filename_argument(path), True)]
-            handle = subprocess.Popen(cmd, stderr=compat_subprocess_get_DEVNULL(), stdout=subprocess.PIPE)
+            handle = subprocess.Popen(cmd, stderr=compat_subprocess_get_DEVNULL(), stdout=subprocess.PIPE, stdin=subprocess.PIPE)
             output = handle.communicate()[0]
             if handle.wait() != 0:
                 return None