Special-case value.Length == 0 in LastIndexOfOrdinal

Special-case value.Length == 0 in LastIndexOfOrdinal

With https://github.com/dotnet/coreclr/pull/579, I added to IndexOfOrdinal a check for value.Length == 0, to avoid returning bad data in that case.  But in doing so, and in neglecting to add a similar check to LastIndexOfOrdinal (which is currently implemented in terms of IndexOfOrdinal), I introduced an infinite loop into LastIndexOfOrdinal when value.Length == 0.  Fixing by adding a similar special-case.