`lfs_fs_gc` causes nllptr deref when called before mounting

[diggit]

It happens here:

littlefs/lfs.c

Lines 5211 to 5213 in 6cb4e86

	// available
	if (lfs->cfg->compact_thresh
	< lfs->cfg->block_size - lfs->cfg->prog_size) {

because lfs->cfg is set in lfs_init called from lfs_mount.

Is the missing assert for mounted state intentional?