Ensure that NLSF cannot be negative when computing a min distance between them b/31607432 Signed-off-by: Jean-Marc Valin <jmvalin@jmvalin.ca> (cherry picked from commit d9d5ac4027c5ee1da2ff1c6572ae3b35a845db19) Change-Id: I506598e403ab03118da6f6cc1331d84e0de2e655