lmdb: hack to avoid windows compilation error involving mdb_filehandle_t
lmdb: hack to avoid windows compilation error involving mdb_filehandle_t

Relates to #33

The type is a pointer on Windows and comparing a pointer to the value -1
is non-trivial in Go.  The comparison has been replaced with a concise
but delicate hack that is untested as of yet.

bmatsuo/lmdb-go,bmatsuo/lmdb-go