fifo: use av_freep()

With this change libavutil uses av_freep() everywhere where it makes sense.
Remaining av_free() use in it has the used pointer become inaccessible quickly
soo zeroing makes no sense.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavutil/fifo.c

@@ -38,7 +38,7 @@ AVFifoBuffer *av_fifo_alloc(unsigned int size)
 void av_fifo_free(AVFifoBuffer *f)
 {
     if(f){
-        av_free(f->buffer);
+        av_freep(&f->buffer);
         av_free(f);
     }
 }