Re-port dump_bitset_clear to -fsanitize=undefined

* src/pdumper.c (dump_bitset_clear): Skip the memset if the
size is zero, because in that case the destination might be NULL.
This fixes a bug introduced in 2019-07-26T06:17:52Zeggert@cs.ucla.edu.
Add a comment to make the bug less likely to reoccur.

1 files changed, 4 insertions, 1 deletions

diff --git a/src/pdumper.c b/src/pdumper.c
index e0ddc1c8088..326a346a632 100644
--- a/src/pdumper.c
+++ b/src/pdumper.c
@@ -4931,7 +4931,10 @@ dump_bitset_set_bit (struct dump_bitset *bitset, size_t bit_number)
 static void
 dump_bitset_clear (struct dump_bitset *bitset)
 {
-  memset (bitset->bits, 0, bitset->number_words * sizeof bitset->bits[0]);
+  /* Skip the memset if bitset->number_words == 0, because then bitset->bits
+     might be NULL and the memset would have undefined behavior.  */
+  if (bitset->number_words)
+    memset (bitset->bits, 0, bitset->number_words * sizeof bitset->bits[0]);
 }
 
 struct pdumper_loaded_dump_private