Fix possible freeze if opcode.txt file does not exist on .debug send opcode

I am no ifstream expert but I think handling it like this looks safer according to http://gehrcke.de/2011/06/reading-files-in-c-using-ifstream-dealing-correctly-with-badbit-failbit-eofbit-and-perror/

Originally inspired by https://github.com/TrinityCore/TrinityCore/commit/35ef163703164a8190473ee8eb2ece32a9565045