2) Assert that the source operand is not special. Prevent resulting assert
failure (harmless) by initializing flags before calling mpd_qshiftr_inplace.
3) Save a couple of instructions (mpd_zerocoeff already sets digits and len).
Reorder initialization to match the order in the mpd_t struct.
in libmpdec to prevent undefined behavior if an invalid context is used. This
cannot occur for the _decimal module since user input for the context is
validated.
