Use multiplication to avoid undefined behaviour per clang

Newer versions of MSVC with /we4146 don't like putting negative sign behind a
unsigned number as https://github.com/harfbuzz/harfbuzz/pull/2069
That however have made https://crbug.com/1050424 this complain:
  src/hb-ot-color-sbix-table.hh:304:28: runtime error: negation of -2147483648 cannot be represented in type 'int';
                                        cast to an unsigned type to negate this value to itself
which apparently can be fixed using this change.

Let's see if this won't make another ubsan complain!

src/hb-ot-color-sbix-table.hh

@@ -301,7 +301,7 @@ struct sbix
       extents->x_bearing = x_offset;
       extents->y_bearing = png.IHDR.height + y_offset;
       extents->width     = png.IHDR.width;
-      extents->height    = -static_cast<int>(png.IHDR.height);
+      extents->height    = -1 * png.IHDR.height;
 
       /* Convert to font units. */
       if (strike_ppem)