* tools/tiffcp.c: error out cleanly in cpContig2SeparateByRow and
cpSeparate2ContigByRow if BitsPerSample != 8 to avoid heap based overflow.
Fixes http://bugzilla.maptools.org/show_bug.cgi?id=2656 and
http://bugzilla.maptools.org/show_bug.cgi?id=2657