[hb-cairo] Silence warning when building with FreeType

In file included from ../util/hb-view.cc:33:
In file included from ../util/view-cairo.hh:32:
../util/helper-cairo.hh:102:7: warning: variable 'cairo_face' is used uninitialized whenever 'if' condition is false [-Wsometimes-uninitialized]
  if (use_hb_draw)
      ^~~~~~~~~~~
../util/helper-cairo.hh:129:64: note: uninitialized use occurs here
  cairo_scaled_font_t *scaled_font = cairo_scaled_font_create (cairo_face,
                                                               ^~~~~~~~~~
../util/helper-cairo.hh:102:3: note: remove the 'if' if its condition is always true
  if (use_hb_draw)
  ^~~~~~~~~~~~~~~~
../util/helper-cairo.hh:101:32: note: initialize the variable 'cairo_face' to silence this warning
  cairo_font_face_t *cairo_face;
                               ^
                                = nullptr

We know that cairo_face will always be assigned since use_hb_draw will
always be true, but the compiler does not know that.

util/helper-cairo.hh

@@ -98,7 +98,7 @@ helper_cairo_create_scaled_font (const font_options_t *font_opts)
 #endif
 
 
-  cairo_font_face_t *cairo_face;
+  cairo_font_face_t *cairo_face = nullptr;
   if (use_hb_draw)
   {
     cairo_face = hb_cairo_font_face_create_for_font (font);