aft: Avoid invalid read.

A short chunk table is fatal for all audio formats except aac,
which employs dynamic chunks. The below valgrind spat was found when
para_server tried to open an aac audio file. Setting afhi->chunk_table
to NULL in this case should fix it.

==17667== Invalid read of size 4
==17667==    at 0x805A862: write_u32 (portable_io.h:95)
==17667==    by 0x805A862: save_chunk_table (aft.c:402)
==17667==    by 0x805A862: save_chunk_table (aft.c:395)
==17667==    by 0x805DDE6: save_afd (aft.c:616)
==17667==    by 0x805DDE6: open_and_update_audio_file (aft.c:1113)
==17667==    by 0x8058AA2: open_next_audio_file (afs.c:425)
==17667==    by 0x8058AA2: execute_server_command (afs.c:867)
==17667==    by 0x8058AA2: command_post_select.part.0 (afs.c:921)
==17667==    by 0x8063062: call_post_select (sched.c:80)
==17667==    by 0x8063062: sched_post_select (sched.c:106)
==17667==    by 0x8063062: schedule (sched.c:159)
==17667==    by 0x8059643: afs_init (afs.c:1006)
==17667==    by 0x804D747: init_afs (server.c:529)
==17667==    by 0x804D747: server_init (server.c:601)
==17667==    by 0x804D747: main (server.c:690)
==17667==  Address 0x4d7dcd0 is 0 bytes after a block of size 40 alloc'd
==17667==    at 0x40365E2: malloc (vg_replace_malloc.c:309)
==17667==    by 0x8053AB6: para_malloc (string.c:63)
==17667==    by 0x805B20D: load_chunk_table (aft.c:415)
==17667==    by 0x805DD65: open_and_update_audio_file (aft.c:1103)
==17667==    by 0x8058AA2: open_next_audio_file (afs.c:425)
==17667==    by 0x8058AA2: execute_server_command (afs.c:867)
==17667==    by 0x8058AA2: command_post_select.part.0 (afs.c:921)
==17667==    by 0x8063062: call_post_select (sched.c:80)
==17667==    by 0x8063062: sched_post_select (sched.c:106)
==17667==    by 0x8063062: schedule (sched.c:159)
==17667==    by 0x8059643: afs_init (afs.c:1006)
==17667==    by 0x804D747: init_afs (server.c:529)
==17667==    by 0x804D747: server_init (server.c:601)
==17667==    by 0x804D747: main (server.c:690)