Set frame defaults in try_decode_frame.

Set frame defaults in try_decode_frame.
Fix:
==22211== Conditional jump or move depends on uninitialised value(s)
==22211==    at 0x811E692: ff_print_debug_info (mpegvideo.c:1137)
==22211==    by 0x82E6A7B: decode_frame (h264.c:7897)
==22211==    by 0x80F2823: avcodec_decode_video2 (utils.c:564)
[...]
==22211==  Uninitialised value was created by a stack allocation
==22211==    at 0x8064AA3: try_decode_frame (utils.c:1845)

Originally committed as revision 18483 to svn://svn.ffmpeg.org/ffmpeg/trunk