Fixes TypeError when attempting to call copyFrom on an undefined comment.
Fixes TypeError when attempting to call copyFrom on an undefined comment.

SierraSoftworks/typedoc,PolymerLabs/typedoc,sebastian-lenz/typedoc,innerverse/typedoc,sebastian-lenz/typedoc,aciccarello/typedoc,innerverse/typedoc,SierraSoftworks/typedoc,TypeStrong/typedoc,aciccarello/typedoc,TypeStrong/typedoc,PolymerLabs/typedoc,adamhickey/typedoc,innerverse/typedoc,adamhickey/typedoc,TypeStrong/typedoc,dominic31/typedoc,sebastian-lenz/typedoc,SierraSoftworks/typedoc,adamhickey/typedoc,dominic31/typedoc,sebilasse/typedoc,sebilasse/typedoc,aciccarello/typedoc,dominic31/typedoc,PolymerLabs/typedoc,sebilasse/typedoc