mm/cirbuf: Fix cannot continue read when tail > head

when head=0, tail has not yet rolled back, and at this time tail>head still has data to read

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

mm/circbuf/circbuf.c

@@ -301,11 +301,16 @@ ssize_t circbuf_peekat(FAR struct circbuf_s *circ, size_t pos,
 
   DEBUGASSERT(circ);
 
-  if (!circ->size || pos >= circ->head)
+  if (!circ->size)
     {
       return 0;
     }
 
+  if (circ->head - pos > circ->head - circ->tail)
+    {
+      pos = circ->tail;
+    }
+
   len = circ->head - pos;
   off = pos % circ->size;