if for a semi definite Matrix normSquared(f) is negative zero i.e. e.g. -1e-16...

if for a semi definite Matrix normSquared(f) is negative zero i.e. e.g. -1e-16 operator() will return -nan. Now normSquared returns 0 in such a case and throws an exception for 'really' negative values

[[Imported from SVN: r6070]]