xbps_[gs]et_cachedir(): ensure we don't copy more than buffer's size including NUL.
xbps_[gs]et_cachedir(): ensure we don't copy more than buffer's size including NUL.

datenwolf/xbps,stpx/xbps,ebfe/xbps,ebfe/xbps,stpx/xbps,datenwolf/xbps,datenwolf/xbps,ebfe/xbps,stpx/xbps