Utils: Fix a bug in the linear transformation code.

Fix a bug where an incorrect result would be computed if you used the
linear transformation code to do a reverse transformation (from B's
domain into A's domain) when the scaler fraction was negative.

Change-Id: I8e5f109314d235a177ab41f65d3c4cd08cff78be
Signed-off-by: John Grossman <johngro@google.com>