telephony: fix maximum limit of destination string
telephony: fix maximum limit of destination string

Trivial fix, the limit should be the remained space in preallocated buffer.

jusa/ohm-plugins-misc,jusa/ohm-plugins-misc,jusa/ohm-plugins-misc,maemo-foss/maemo-multimedia-ohm-plugins-misc,tswindell/ohm-plugins-misc,maemo-foss/maemo-multimedia-ohm-plugins-misc,tswindell/ohm-plugins-misc,tswindell/ohm-plugins-misc,jusa/ohm-plugins-misc,maemo-foss/maemo-multimedia-ohm-plugins-misc