[clang] Fix static analyzer concerns (llvm#110243)

It seems in checkOpenMPIterationSpace `OrderedLoopCountExpr` can also be
null, so check before dereferencing.

Author: Fznamznon

clang/lib/Sema/SemaOpenMP.cpp

@@ -9195,7 +9195,7 @@ static bool checkOpenMPIterationSpace(
         SemaRef.Diag(CollapseLoopCountExpr->getExprLoc(),
                      diag::note_omp_collapse_ordered_expr)
             << 0 << CollapseLoopCountExpr->getSourceRange();
-      else
+      else if (OrderedLoopCountExpr)
         SemaRef.Diag(OrderedLoopCountExpr->getExprLoc(),
                      diag::note_omp_collapse_ordered_expr)
             << 1 << OrderedLoopCountExpr->getSourceRange();