valgrind: use OUString::operator to access into string

address some out of bounds oddness seen with
rtl_ustr_getlength_heap_buffer_overflow.sample

Change-Id: I5a9772de9607644f43e74174f73053d292ca7cc0
Reviewed-on: https://gerrit.libreoffice.org/55722Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

basic/source/inc/scanner.hxx

@@ -34,8 +34,8 @@ class SbiScanner
 {
     OUString   aBuf;             // input buffer
     OUString   aLine;
-    const sal_Unicode* pLine;
-    const sal_Unicode* pSaveLine;
+    sal_Int32 nLineIdx;
+    sal_Int32 nSaveLineIdx;
     StarBASIC* pBasic;                  // instance for error callbacks
 
     void scanAlphanumeric();
@@ -80,8 +80,8 @@ public:
     sal_Int32 GetCol1()             { return nCol1;   }
     void  SetCol1( sal_Int32 n )    { nCol1 = n;      }
     StarBASIC* GetBasic()           { return pBasic;  }
-    void  SaveLine()            { pSaveLine = pLine; }
-    void  RestoreLine()         { pLine = pSaveLine; }
+    void  SaveLine()                { nSaveLineIdx = nLineIdx; }
+    void  RestoreLine()             { nLineIdx = nSaveLineIdx; }
     void  LockColumn();
     void  UnlockColumn();
     bool  DoesColonFollow();