mingw-w64: fix a bug in the installer

The MinGW-W64 installer available at https://sourceforge.net/projects/mingw-w64 has an error which reads

the file has been downloaded incorrectly!

https://sourceforge.net/p/mingw-w64/bugs/558 - reported on August 2016 and supposedly fixed... only it isn't.

https://stackoverflow.com/questions/46455927/mingw-w64-installer-the-file-has-been-downloaded-incorrectly
https://stackoverflow.com/questions/41109422/mingw64-the-file-has-been-downloaded-incorrectly