Rearrange player cleanup code

Since apparently closing stdin and later calling communicate() is no bueno, 
we're just going to rearrange the process finalization code so both cleanup()
and the pipe loop exit conditions point to it.

discord/player.py

@@ -155,7 +155,7 @@ class FFmpegAudio(AudioSource):
         self._pipe_thread: Optional[threading.Thread] = None
 
         if piping:
-            n = f'PopenStdinWriter:{id(self):#x}'
+            n = f'popen-stdin-writer:{id(self):#x}'
             self._stdin = self._process.stdin
             self._pipe_thread = threading.Thread(target=self._pipe_writer, args=(source,), daemon=True, name=n)
             self._pipe_thread.start()
@@ -172,22 +172,7 @@ class FFmpegAudio(AudioSource):
         else:
             return process
 
-    def _pipe_writer(self, source: io.BufferedIOBase) -> None:
-        while self._process:
-            # arbitrarily large read size 
-            data = source.read(8192)
-            if not data:
-                self._stdin.close() # EOF
-                break
-            try:
-                self._stdin.write(data)
-            except Exception:
-                _log.debug('Write error for %s, this is probably not a problem', self, exc_info=True)
-                # at this point the source data is either exhausted or the process is fubar
-                self._stdin.close()
-                break
-
-    def cleanup(self) -> None:
+    def _kill_process(self) -> None:
         proc = self._process
         if proc is MISSING:
             return
@@ -206,7 +191,25 @@ class FFmpegAudio(AudioSource):
         else:
             _log.info('ffmpeg process %s successfully terminated with return code of %s.', proc.pid, proc.returncode)
 
-        self._process = self._stdout = MISSING
+
+    def _pipe_writer(self, source: io.BufferedIOBase) -> None:
+        while self._process:
+            # arbitrarily large read size
+            data = source.read(8192)
+            if not data:
+                self._process.terminate()
+                return
+            try:
+                self._stdin.write(data)
+            except Exception:
+                _log.debug('Write error for %s, this is probably not a problem', self, exc_info=True)
+                # at this point the source data is either exhausted or the process is fubar
+                self._process.terminate()
+                return
+
+    def cleanup(self) -> None:
+        self._kill_process()
+        self._process = self._stdout = self._stdin = MISSING
 
 class FFmpegPCMAudio(FFmpegAudio):
     """An audio source from FFmpeg (or AVConv).