mm/circbuf: fix compile warning

circbuf/circbuf.c:133:14: warning: 'len' may be used uninitialized in this function [-Wmaybe-uninitialized]
  133 |   circ->head = len;
      |   ~~~~~~~~~~~^~~~~
circbuf/circbuf.c:106:10: note: 'len' was declared here
  106 |   size_t len;
      |          ^~~

Change-Id: I60e42b88e98838f5654d84b70f109a4df5a7ff26
Signed-off-by: chao.an <anchao@xiaomi.com>

mm/circbuf/circbuf.c

@@ -103,7 +103,7 @@ int circbuf_init(FAR struct circbuf_s *circ, FAR void *base, size_t bytes)
 int circbuf_resize(FAR struct circbuf_s *circ, size_t bytes)
 {
   FAR void *tmp = NULL;
-  size_t len;
+  size_t len = 0;
 
   DEBUGASSERT(circ);
   DEBUGASSERT(!circ->external);