pnmdec: use explicit casts to remove const to avoid warning with clang

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/pnmdec.c

@@ -37,8 +37,8 @@ static int pnm_decode_frame(AVCodecContext *avctx, void *data,
     int components, sample_len;
 
     s->bytestream_start =
-    s->bytestream       = buf;
+    s->bytestream       = (uint8_t *)buf;
-    s->bytestream_end   = buf + buf_size;
+    s->bytestream_end   = (uint8_t *)buf + buf_size;
 
     if (ff_pnm_decode_header(avctx, s) < 0)
         return -1;