GDALRasterBand::GetHistogram(): ignore nodata values in the GDT_Byte special case (#5289, patch by nflood, complementary of #4750)

git-svn-id: https://svn.osgeo.org/gdal/trunk@26590 f0d54148-0727-0410-94bb-9a71ac55c965
Merge branch 'trunk' into trunk_travis_macosx