Handle incorrect usage of uniform_real_distribution()

uniform_real_distribution(a,b) should be called with a < b, otherwise
the result is undefined. Currently, when called with both zero, it
gets stuck in a loop. Not sure if a blunt assert() would be the right
thing here, so I just return a if a >= b.

Change-Id: I769688c7192bd02bad24ad597948984db56dd4fc
Don't call comphelper::rng::uniform_real_distribution(a,b) with a==b

Change-Id: I95a3ac8da83e02001de13af8d31a04fe0d654dc0