mutt segfaults in cmd_parse_expunge

[Dan Watkins]

A short backtrace:

Thread 1 (Thread 0x7ffff7f948c0 (LWP 3606)):
#0 0x000055555562ee0e in cmd_parse_expunge (s=0x555555a1a8a2 "42 EXPUNGE", idata=0x555555a13f60) at ../../../imap/command.c:209
#1 0x000055555562ee0e in cmd_handle_untagged (idata=0x555555a13f60) at ../../../imap/command.c:719
#2 0x000055555562ee0e in imap_cmd_step (idata=idata@entry=0x555555a13f60) at ../../../imap/command.c:837
#3 0x000055555562fb38 in imap_exec (idata=idata@entry=0x555555a13f60, cmdstr=cmdstr@entry=0x5555556538d6 "EXPUNGE", flags=flags@entry=0) at ../../../imap/command.c:941
#4 0x00005555556327fc in imap_sync_mailbox (ctx=ctx@entry=0x555555a04df0, expunge=expunge@entry=1) at ../../../imap/imap.c:1348
#5 0x00005555555cbe7b in mx_sync_mailbox (ctx=ctx@entry=0x555555a04df0, index_hint=index_hint@entry=0x7fffffffce24) at ../../mx.c:1239
#6 0x0000555555597234 in mutt_index_menu () at ../../curs_main.c:1729
#7 0x000055555557274a in main (argc=1, argv=<optimised out>, env=<optimised out>) at ../../main.c:920

(Fuller backtrace attached.)

[Dan Watkins (oddbloke)]

This is mutt 1.8.3+neomutt20170609-2build1 on artful.