Port dump_bitset_clear to -fsanitize=undefined

* src/pdumper.c (dump_bitset_clear):
Pacify -fsanitize=undefined by avoiding memset (NULL, x, 0),
which strictly speaking has undefined behavior although it
works on all production platforms I know.

src/pdumper.c

@@ -4923,7 +4923,8 @@ static void
 dump_bitset_clear (struct dump_bitset *bitset)
 {
   int xword_size = sizeof (bitset->bits[0]);
-  memset (bitset->bits, 0, bitset->number_words * xword_size);
+  if (bitset->number_words)
+    memset (bitset->bits, 0, bitset->number_words * xword_size);
 }
 
 struct pdumper_loaded_dump_private