reduce the chances of a race condition in mplayer code

Not perfect, it may still happen.

https://anki.tenderapp.com/discussions/ankidesktop/39832-an-error-occurred-audio

qt/aqt/sound.py

@@ -393,6 +393,7 @@ class SimpleMplayerSlaveModePlayer(SimpleMplayerPlayer):
 
         The trailing newline is automatically added."""
         str_args = [str(x) for x in args]
+        if self._process:
             self._process.stdin.write(" ".join(str_args).encode("utf8") + b"\n")
             self._process.stdin.flush()