h264: increase reference poc list from 16 to 32.

Interlaced images can have 32 references (16 per field), so limiting the
array size to 16 leads to invalid writes.

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org

libavcodec/mpegvideo.h

@@ -126,7 +126,7 @@ typedef struct Picture{
     int pic_id;                 /**< h264 pic_num (short -> no wrap version of pic_num,
                                      pic_num & max_pic_num; long -> long_pic_num) */
     int long_ref;               ///< 1->long term reference 0->short term reference
-    int ref_poc[2][2][16];      ///< h264 POCs of the frames used as reference (FIXME need per slice)
+    int ref_poc[2][2][32];      ///< h264 POCs of the frames used as reference (FIXME need per slice)
     int ref_count[2][2];        ///< number of entries in ref_poc              (FIXME need per slice)
     int mbaff;                  ///< h264 1 -> MBAFF frame 0-> not MBAFF
     int field_picture;          ///< whether or not the picture was encoded in separate fields