Fix two obvious mistakes from 8c37bafbc5adc0c79f218f9db2dc25ca8e683125.

Add asserts to make sure the calls to ScFormulaCell::CreateCellGroup() that
now don't take the nStart parameter any more (but assume that the cell for
which this methoid is called is the start one) match the old ones in semantics.

Change-Id: I50cb0cdaf620253881225baf39ae9cd935917b27