[ruamoko] Handle missing fonts correctly

Carrying on as if the missing font had been loaded leads to way too many
issues for it to be a good thing (not that that really needs to be
said). Fixes the segfaults in my test scene.

libs/ruamoko/rua_gui.c

@@ -223,10 +223,14 @@ bi (Font_Load)
 	const char *font_path = P_GSTRING (pr, 0);
 	int         font_size = P_INT (pr, 1);
 
+	R_INT (pr) = 0;
+
 	QFile      *font_file = QFS_FOpenFile (font_path);
 	font_t     *font = Font_Load (font_file, font_size);
+	if (font) {
 		R_INT (pr) = alloc_font (res, font);
 	}
+}
 
 bi (Passage_New)
 {