Check the magic number of a decoded file

Checking the magic number of a file upon decoding is necessary,
because if the file is corrupt and we don't check it, we will
fall back to a simple copy of the corrupted file. Latin IME
would realize this and would not crash, but would not use the
corrupted dictionary. If this happened to be a main dictionary,
then the user would lose the ability to use the correct
built-in dictionary.

Not the same, but kinda similar to
Bug: 5223031

Change-Id: Ic2783dc9dd5f3dcf2865623d9452765fe3778db7