bugfix for MatMatMultSymbolic_MPIAIJ_MPIDense() when Bbn1 = 0.
Reported-by: Pierre Jolivet pierre@joliv.et Bb (column block size) cannot be zero; it leads to infinite loop in MatMatMultNumeric_MPIAIJ_MPIDense() with n=0
Reported-by: Pierre Jolivet pierre@joliv.et Bb (column block size) cannot be zero; it leads to infinite loop in MatMatMultNumeric_MPIAIJ_MPIDense() with n=0