Increase size of range request in getrange

32 bit builds don't have a big enough long to capture
the same range as a 64 bit build.  If we use "long long"
we get proper size limits everywhere.

Also updates size of unsigned comparison to fit new size of `end`.

Fixes #1981
Return empty string if GETRANGE of empty string

Previously, GETRANGE of a key containing nothing ("")
would allocate a large (size_t)-1 return value causing
crashes on 32bit builds when it tried to allocate the
4 GB return string.
Merge pull request #1982 from mattsta/fix-getrange-32bit

Increase size of range request in getrange