libchewing: Negative-size-param in ChewingKillChar

Detailed report: https://clusterfuzz-external.appspot.com/testcase?key=6311985729765376 Fuzzer: libFuzzerlibchewingchewingdynamicconfigfuzzer Job Type: libfuzzerasanlibchewing Platform Id: linux Crash Type: Negative-size-param Crash Address: Crash State: ChewingKillChar KillFromLeft...