resolved fdo#63161 out of bounds string access

Introduced with c7709b46
String to OUString where previous String::GetChar() handled this.

Change-Id: I9253df1af7498e2d7a8ea8077ec6b369697ad44e

svl/source/numbers/zforfind.hxx

@@ -218,6 +218,10 @@ private:
                                        const OUString& rString,
                                        sal_Int32 nPos )
         {
+            if (rWhat.isEmpty() || rString.getLength() <= nPos)
+            {
+                return false;
+            }
             // mostly used with one character
             if ( rWhat[ 0 ] != rString[ nPos ] )
             {