Fixed 'File Being Used in Other Process' Bug

Added more validation code and another function call to release the file
streams whenever switching modes (encrypt/decrypt).
Fixed Bug, and Added Extra Security Measure

Fixed a bug where whenever the user played a wav file in decrypt mode,
then changed the mode from decrypt to encrypt, and tried to inject a
message into the wav file previously played in decrypt mode, an error
would say that the wav file was being used in another process. I also
made it so that 300 characters are always displayed when decrypting
(padded the real message to display with spaces when the correct
password is entered).