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: I18d8a1a6aceb54d36e18f77cb385e3b505e969f5
1 file changed