The vtkLookupTableLogRange function had conditions for when
rmax and rmin were both positive or both negative, but not for
when they spanned zero or were both equal to zero.  This was
caught by clang's static analyzer.  Values are clamped to
VTK_DBL_MIN (absolute) or 1e-6 (relative).

Change-Id: I02a52a574e7bcff8c364fb40ea21bea84f37e518