Fix the "Install" button stuck in "Installing" when font install is successful

Currently in GNOME Fonts, when installing a font, the button always stays stuck in "Installing", even though it's usually immediately done.

This is the first C code I ever write, and I don't know much about the structure of the code, so forgive any mistakes.

As far as I understand, I called the function that refreshes the button with a NULL error in the success case.