Fix: autoset bx/by/bz when set nx/ny/nz manually

[hongriTianqi]

Fix #2726

The bug in #2726 happens because 0 is put in the denominator:
This happens only in LCAO calculation. In pw, bx = by = bz = 1 is set in input.cpp line 2693. It happens in

void ESolver_FP::Init
{
...
            if (inp.nx * inp.ny * inp.nz == 0)
            this->pw_rho->initgrids(inp.ref_cell_factor * cell.lat0, cell.latvec, inp.ecutrho);
	    else
            this->pw_rho->initgrids(inp.ref_cell_factor * cell.lat0, cell.latvec, inp.nx, inp.ny, inp.nz);
...
}

where, if users have set nx, ny, nz, it will go to the second branch, then in the following function:

virtual void initgrids(
    const double lat0_in,
    const ModuleBase::Matrix3 latvec_in, // Unitcell lattice vectors
    const int nx_in, int ny_in, int nz_in
    )
{
...
        if(this->nx%this->bx != 0) this->nx += (this->bx - this->nx % this->bx);
        if(this->ny%this->by != 0) this->ny += (this->by - this->ny % this->by);
        if(this->nz%this->bz != 0) this->nz += (this->bz - this->nz % this->bz);
...
}

bx, by, bz are now zero, aborting the whole process.