assert fail in ChewingIsChiAt

[kcwu]

original report https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=207

How to reproduce

$ ./autogen.sh
$ ./configure
$ make clean check
$ ./test/stress -loop 1 -extra -verbose -file 207-assert
stress: chewingutil.c:944: ChewingIsChiAt: Assertion `0 <= chiSymbolCursor' failed.

reduced test case

$ xxd 207-assert
00000000: c20b 3030 3030 3030 3030 3030 3030 3030  ..00000000000000
00000010: 593a 3030 0b59 3a0b 06cb 0105 0505       Y:00.Y:.......