[FIX] Trust: use Amount's public api
[FIX] Trust: use Amount's public api

Use public method to check if value is zero, not private
field _offset.
Internals of Amount class was changed and it does not have
_offset field now.

xdv/ripple-client,xdv/ripple-client,h0vhannes/ripple-client,mrajvanshy/ripple-client,MatthewPhinney/ripple-client,arturomc/ripple-client,darkdarkdragon/ripple-client,darkdarkdragon/ripple-client,yongsoo/ripple-client,wangbibo/ripple-client,mrajvanshy/ripple-client,Madsn/ripple-client,xdv/ripple-client,wangbibo/ripple-client,yongsoo/ripple-client,vhpoet/ripple-client,wangbibo/ripple-client,darkdarkdragon/ripple-client,darkdarkdragon/ripple-client,h0vhannes/ripple-client,vhpoet/ripple-client,vhpoet/ripple-client,mrajvanshy/ripple-client,arturomc/ripple-client,Madsn/ripple-client,h0vhannes/ripple-client,Madsn/ripple-client,MatthewPhinney/ripple-client,MatthewPhinney/ripple-client,wangbibo/ripple-client,MatthewPhinney/ripple-client,vhpoet/ripple-client,h0vhannes/ripple-client,arturomc/ripple-client,yongsoo/ripple-client,arturomc/ripple-client,xdv/ripple-client,yongsoo/ripple-client,Madsn/ripple-client,mrajvanshy/ripple-client