fs/mmap: Fix build warning with [-Wmaybe-uninitialized].

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

fs/mmap/fs_rammap.c

@@ -52,7 +52,7 @@ static int unmap_rammap(FAR struct task_group_s *group,
                         FAR void *start,
                         size_t length)
 {
-  FAR void *newaddr;
+  FAR void *newaddr = NULL;
   off_t offset;
   bool kernel = entry->priv.i;
   int ret = OK;