Explicitly return a value when command should silently fail

Summary:
This fixes the following compilation issue:

./../source/chan.c:91:6: error: non-void function 'do_math'
should return a value [-Wreturn-type] return;

Test Plan: gmake

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1646

Differential Revision: https://devcentral.nasqueron.org/D2395