don't use opencl with deleted cells

Without this testFormulaRefUpdateDeleteAndShiftLeft2 fails with OpenCL
enabled (and the mnOpenCLMinimumFormulaGroupSize check disabled).
If I'm getting it right, anything working with #REF! (=invalid reference)
should result in #REF! too, but without this fix the function effectively
converted the #REF! to an empty cell.

Change-Id: Icdda64ad751d7c0e9c0d5995673e4fb30c4c9ac4
Reviewed-on: https://gerrit.libreoffice.org/55672Reviewed-by: Michael Meeks <michael.meeks@collabora.com>
Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Tor Lillqvist <tml@collabora.com>

sc/source/core/data/grouptokenconverter.cxx

@@ -116,6 +116,8 @@ bool ScGroupTokenConverter::convert( const ScTokenArray& rCode, sc::FormulaLogge
             case svSingleRef:
             {
                 ScSingleRefData aRef = *p->GetSingleRef();
+                if( aRef.IsDeleted())
+                    return false;
                 ScAddress aRefPos = aRef.toAbs(mrPos);
                 if (aRef.IsRowRel())
                 {
@@ -178,6 +180,8 @@ bool ScGroupTokenConverter::convert( const ScTokenArray& rCode, sc::FormulaLogge
 
 #if 0
                 ScComplexRefData aRef = *p->GetDoubleRef();
+                if( aRef.IsDeleted())
+                    return false;
                 ScRange aAbs = aRef.toAbs(mrPos);
 
                 // Multiple sheets not handled by vector/matrix.