Change assert to runtime check.

I had a system that was constantly hitting this assert, after making
this change it recovered immediately.

lfs.c

@@ -29,8 +29,7 @@ static int lfs_bd_read(lfs_t *lfs,
         lfs_block_t block, lfs_off_t off,
         void *buffer, lfs_size_t size) {
     uint8_t *data = buffer;
-    LFS_ASSERT(block != LFS_BLOCK_NULL);
-    if (off+size > lfs->cfg->block_size) {
+    if ((off+size > lfs->cfg->block_size) || (block == LFS_BLOCK_NULL)) {
         return LFS_ERR_CORRUPT;
     }