font_load_ft.c: fix possible crash with recent Fontconfig
Calling FcFontMatch with last argument NULL triggers an assertion
failure in latest libfontconfig (2.9.0). Change the code to use a
dummy address, like another nearby call to FcFontMatch already did.
Whether the code triggering the assertion failure was run depended on
font settings and/or available system fonts (the code looks like you'd
have to get a non-scalable font first). Thus it didn't usually happen
even with problematic libfontconfig version.