Don't use output rArea when TableLayouter::getCellArea returns false

...when xCell->isMerged, as happens during CppunitTest_sd_filters_test.
aCellArea will be "empty" (i.e., getMinX()==SAL_MAX_INT32 etc.) and the
aCellRect.Move calculation will overflow for 32-bit long.

Change-Id: I12649234c060edd9f2cab1792cf75335ae22b730