create_bits(): Cast the result of height * stride to size_t

In create_bits() both height and stride are ints, so the result is
also an int, which will overflow if height or stride are big enough
and size_t is bigger than int.

This patch simply casts height to size_t to prevent these overflows,
which prevents the crash in:

    https://bugzilla.redhat.com/show_bug.cgi?id=972647

It's not even close to fixing the full problem of supporting big
images in pixman.

See also

    https://bugs.freedesktop.org/show_bug.cgi?id=69014