mm/circbuf: skip buffer content according position in circbuf_peekat

The circbuf_peekat should copy valid content from the specfied position
of buffer, so skip the area from this position to the tail.

Signed-off-by: dongjiuzhu1 <dongjiuzhu1@xiaomi.com>

mm/circbuf/circbuf.c

@@ -301,12 +301,12 @@ ssize_t circbuf_peekat(FAR struct circbuf_s *circ, size_t pos,
 
   DEBUGASSERT(circ);
 
-  if (!circ->size)
+  if (!circ->size || pos >= circ->head)
     {
       return 0;
     }
 
-  len = circbuf_used(circ);
+  len = circ->head - pos;
   off = pos % circ->size;
 
   if (bytes > len)