Currently, a.voting_power of users who haven't voted for more than 5 days but had some financial transaction (e.g., claim reward) is set to 0, which no longer represents the voting power at the a.last_vote_time.

This fix returns the previous a.voting power when this reset happens (unless current_mana is 0, case 2 below).

This fix is safe cause vp_t1 = 0 can occur only in the following two cases:

1) 5 days passed
current voting power (calculated from a.voting_power and a.last_vote_time) will be 100% either way. But this fix is more informative, since it returns the last voting power.

2) vp_t2u = 0
L2085 return 0 (same as before)

fix zero voting power