fdo#43534: Reset tab index for SingleRefToVars() call.

Otherwise that method will set #REF! to the cell when it sees the tab
index being -1.  We don't use that index in that code, so it's safe to
do this.

sc/source/core/tool/interpr1.cxx

@@ -2306,7 +2306,13 @@ void ScInterpreter::ScCellExternal()
     SCCOL nCol;
     SCROW nRow;
     SCTAB nTab;
+    aRef.nTab = 0; // -1 for external ref. Plus we don't use this.
     SingleRefToVars(aRef, nCol, nRow, nTab);
+    if (nGlobalError)
+    {
+        PushIllegalParameter();
+        return;
+    }
 
     ScCellKeywordTranslator::transKeyword(aInfoType, ScGlobal::GetLocale(), ocCell);