Fix duplicate frame thread pointer freeing on close

This solves an issue where frame thread pointers are freed twice, by switching to dav1d_freep_aligned() to check whether they are valid on close. This error only seems to occur on certain streams and/or configurations, but multiple users have been able to reproduce it (personally, with http://download.opencontent.netflix.com.s3.amazonaws.com/AV1/Chimera/Chimera-AV1-8bit-480x270-552kbps.ivf).

Fixes the crash in #11 (closed).

src/lib.c

@@ -221,8 +221,8 @@ void dav1d_close(Dav1dContext *const c) {
             pthread_mutex_unlock(&f->frame_thread.td.lock);
             pthread_join(f->frame_thread.td.thread, NULL);
             freep(&f->frame_thread.b);
-            dav1d_free_aligned(&f->frame_thread.pal_idx);
-            dav1d_free_aligned(&f->frame_thread.cf);
+            dav1d_freep_aligned(&f->frame_thread.pal_idx);
+            dav1d_freep_aligned(&f->frame_thread.cf);
             freep(&f->frame_thread.tile_start_off);
             freep(&f->frame_thread.pal);
             freep(&f->frame_thread.cbi);