UBSan: SvxColorItem vs. SvxBrushItem confusion

Assume RES_CHRATR_BACKGROUND shall indeed be an SvxBrushItem while
SID_ATTR_CHAR_COLOR_BACKGROUND shall indeed be an SvxColorItem.  (This probably
never caused trouble in practice as the member vars of SvxColorItem happen to be
a prefix of the member vars of SvxBrushItem.)

Change-Id: Ifb2a137ad89de6e03da3b61dd17bcd8882cce2d7