HAWQ-1573. Clear debug_query_string in proc_exit to avoid crash
HAWQ-1573. Clear debug_query_string in proc_exit to avoid crash

Backport following commit from PostgreSQL:
    commit e1eb7c81192bec3735eed3228202b400f31c8010
    Author: Tom Lane <tgl@sss.pgh.pa.us>
    Date:   Sat Mar 20 00:58:21 2010 +0000

Report from Xiaowen Zheng <xiaowen.zxw@alibaba-inc.com>

jianlirong/incubator-hawq,lavjain/incubator-hawq,jianlirong/incubator-hawq,jianlirong/incubator-hawq,jianlirong/incubator-hawq,jianlirong/incubator-hawq,jianlirong/incubator-hawq,lavjain/incubator-hawq,jianlirong/incubator-hawq,lavjain/incubator-hawq,lavjain/incubator-hawq,lavjain/incubator-hawq,jianlirong/incubator-hawq,lavjain/incubator-hawq,lavjain/incubator-hawq,lavjain/incubator-hawq