Remove useless casts.

Patch by avcoder ffmpeg at gmail d0t com

Originally committed as revision 14153 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/pngenc.c

@@ -232,7 +232,7 @@ static int png_write_row(PNGEncContext *s, const uint8_t *data, int size)
 static int encode_frame(AVCodecContext *avctx, unsigned char *buf, int buf_size, void *data){
     PNGEncContext *s = avctx->priv_data;
     AVFrame *pict = data;
-    AVFrame * const p= (AVFrame*)&s->picture;
+    AVFrame * const p= &s->picture;
     int bit_depth, color_type, y, len, row_size, ret, is_progressive;
     int bits_per_pixel, pass_row_size;
     int compression_level;
@@ -425,8 +425,8 @@ static int encode_frame(AVCodecContext *avctx, unsigned char *buf, int buf_size,
 static av_cold int png_enc_init(AVCodecContext *avctx){
     PNGEncContext *s = avctx->priv_data;
 
-    avcodec_get_frame_defaults((AVFrame*)&s->picture);
-    avctx->coded_frame= (AVFrame*)&s->picture;
+    avcodec_get_frame_defaults(&s->picture);
+    avctx->coded_frame= &s->picture;
     dsputil_init(&s->dsp, avctx);
 
     s->filter_type = av_clip(avctx->prediction_method, PNG_FILTER_VALUE_NONE, PNG_FILTER_VALUE_MIXED);