Fix crash caused by an unsigned overflow.
Fix crash caused by an unsigned overflow.

Thanks to Joseph Bisch for discovery.
This was an artifact of converting ZNC to use CMessage. Released
versions are not affected.

dgw/znc,markusj/znc,DarthGandalf/znc,dgw/znc,Zarthus/znc,DarthGandalf/znc,Zarthus/znc,markusj/znc,GLolol/znc,Zarthus/znc,znc/znc,elyscape/znc,psychon/znc,DarthGandalf/znc,elyscape/znc,psychon/znc,markusj/znc,elyscape/znc,dgw/znc,DarthGandalf/znc,BtbN/znc,Zarthus/znc,dgw/znc,psychon/znc,GLolol/znc,znc/znc,GLolol/znc,elyscape/znc,GLolol/znc,BtbN/znc,dgw/znc,elyscape/znc,GLolol/znc,znc/znc,GLolol/znc,elyscape/znc,psychon/znc,znc/znc,znc/znc,BtbN/znc,markusj/znc,psychon/znc,Zarthus/znc,Zarthus/znc,znc/znc,DarthGandalf/znc,dgw/znc,markusj/znc,psychon/znc,DarthGandalf/znc,BtbN/znc,markusj/znc,BtbN/znc,BtbN/znc