[Polly] Add null pointer check before size retrieval (#174309)

This patch avoids assertion failures by ensuring a null pointer check is
performed before accessing the object's size.
Note: The corresponding test case remains too large even after
reduction, so it has not been included in this patch.

Fixes #174147

polly/lib/Analysis/ScopBuilder.cpp

@@ -3250,6 +3250,9 @@ static bool buildMinMaxAccess(isl::set Set,
   Set = Set.remove_divs();
   polly::simplify(Set);
 
+  if (Set.is_null())
+    return false;
+
   if (unsignedFromIslSize(Set.n_basic_set()) > RunTimeChecksMaxAccessDisjuncts)
     Set = Set.simple_hull();