coa.3: guard XP getters in DataStore_Characters for partial records

GetXPRate / GetRestXPRate did raw arithmetic on character.XP / .XPMax /
.lastLogoutTimestamp, which are nil for own alts seen via guild comm but
never fully scanned. Crashed AccountSummary on Vol'jin - CoA Beta.

DataStore_Characters/DataStore_Characters.toc

@@ -3,7 +3,7 @@
 ## Notes: Stores information about characters (race, level, etc..)
 ## Author: Thaoky (EU-Marécages de Zangar)
 ## X-Edited-By: Exiles (Sub-Net)
-## Version: 3.3.001-coa.2
+## Version: 3.3.001-coa.3
 ## Dependencies: DataStore
 ## OptionalDeps: Ace3
 ## SavedVariables: DataStore_CharactersDB