Fixes a problem that made vim-gnupg open an emtpy file instead of

decrypting the existing one if the filename or path contains pairs
of square brackets

vim-gnupg uses ```glob``` to check if a file exists but glob expands wildcards (including  [ and ]) so, the check returns 'false' even if the file exists if the filename or path is something like ```[0]-Directory/File```.

Using ```filereadable``` avoids this problem.