unset extension, so code path, and guess format do not choose

avm2/flash 9 by default, not sure this is adequate though,
this fixes regression tests.

Originally committed as revision 11588 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/swf.c

@@ -805,7 +805,7 @@ AVOutputFormat avm2_muxer = {
     "avm2",
     "Flash 9 (AVM2) format",
     "application/x-shockwave-flash",
-    "swf",
+    NULL,
     sizeof(SWFContext),
     CODEC_ID_MP3,
     CODEC_ID_FLV1,