atmel-samd: Fix crash when fs mount fails and fix mounting internal flash. It was broken by the update to 1.9.1. Related to #82

atmel-samd/access_vfs.c

@@ -45,6 +45,9 @@
 // The root FS is always at the end of the list.
 static fs_user_mount_t* get_vfs(int index) {
     mp_vfs_mount_t* current_mount = MP_STATE_VM(vfs_mount_table);
+    if (current_mount == NULL) {
+        return NULL;
+    }
     while (current_mount->next != NULL) {
         current_mount = current_mount->next;
     }

atmel-samd/internal_flash.c

@@ -294,7 +294,10 @@ const mp_obj_type_t internal_flash_type = {
 };
 
 void flash_init_vfs(fs_user_mount_t *vfs) {
+    vfs->base.type = &mp_fat_vfs_type;
     vfs->flags |= FSUSER_NATIVE | FSUSER_HAVE_IOCTL;
+    vfs->fatfs.drv = vfs;
+    vfs->fatfs.part = 1; // flash filesystem lives on first partition
     vfs->readblocks[0] = (mp_obj_t)&internal_flash_obj_readblocks_obj;
     vfs->readblocks[1] = (mp_obj_t)&internal_flash_obj;
     vfs->readblocks[2] = (mp_obj_t)internal_flash_read_blocks; // native version