better to set things to NULL instead of random in case of out of mem

Originally committed as revision 7572 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavutil/fifo.c

@@ -24,11 +24,11 @@
 
 int av_fifo_init(AVFifoBuffer *f, int size)
 {
+    f->wptr = f->rptr =
     f->buffer = av_malloc(size);
     if (!f->buffer)
         return -1;
     f->end = f->buffer + size;
-    f->wptr = f->rptr = f->buffer;
     return 0;
 }