Fix an uninitialized field warning in the McParser class
Fix an uninitialized field warning in the McParser class

Summary: g++ with `-Wextra` warned that the `umMsgInfo_` structure wasn't
fully initialized.  This patch zeros out the entire struct.

Test Plan: build mcrouter

Reviewed By: @alikhtarov

Differential Revision: D1810766
yqzhang/mcrouter,evertrue/mcrouter,easyfmxu/mcrouter,synecdoche/mcrouter,evertrue/mcrouter,leitao/mcrouter,easyfmxu/mcrouter,reddit/mcrouter,apinski-cavium/mcrouter,facebook/mcrouter,synecdoche/mcrouter,is00hcw/mcrouter,tempbottle/mcrouter,nvaller/mcrouter,easyfmxu/mcrouter,seem-sky/mcrouter,nvaller/mcrouter,is00hcw/mcrouter,synecdoche/mcrouter,nvaller/mcrouter,seem-sky/mcrouter,tempbottle/mcrouter,reddit/mcrouter,glensc/mcrouter,zhlong73/mcrouter,zhlong73/mcrouter,seem-sky/mcrouter,glensc/mcrouter,zhlong73/mcrouter,evertrue/mcrouter,apinski-cavium/mcrouter,reddit/mcrouter,easyfmxu/mcrouter,glensc/mcrouter,facebook/mcrouter,glensc/mcrouter,evertrue/mcrouter,yqzhang/mcrouter,leitao/mcrouter,reddit/mcrouter,yqzhang/mcrouter,tempbottle/mcrouter,facebook/mcrouter,yqzhang/mcrouter,zhlong73/mcrouter,tempbottle/mcrouter,is00hcw/mcrouter,apinski-cavium/mcrouter,seem-sky/mcrouter,is00hcw/mcrouter,facebook/mcrouter,synecdoche/mcrouter,nvaller/mcrouter,leitao/mcrouter,apinski-cavium/mcrouter,leitao/mcrouter