fix: Prevent users from attaining a negative number of fist bumps.

[SloaneTribble]

When clicking on two separate "shut_up_and_take_my_fistbump" emojis in quick succession, Gratibot informed me that I had sent out six fist bumps and had -1 left to give. This behavior should be prevented, possibly by restructuring the checks performed by Gratibot before administering fist bumps.