mdbx: avoid on-stack allocation/chkstk inside mdbx_reader_check0().
mdbx: avoid on-stack allocation/chkstk inside mdbx_reader_check0().

4 of 17 for https://github.com/leo-yuriev/libmdbx/issues/43

Change-Id: I31a8396ccc5db32cbd1816593861d2ee4325d09f

leo-yuriev/libfpta,leo-yuriev/libfpta