[PATCH] reftable: avoid undefined behaviour breaking t0032

["Carlo Marcelo Arenas Belón" <carenas@gmail.com>]

At least in glibc based systems, memset with a NULL first parameter
will cause a runtime exception.

Avoid doing so by adding a conditional to check for NULL in all three
identically looking functions that were affected.

Signed-off-by: Carlo Marcelo Arenas Belón <carenas@gmail.com>
---
Bug was introduced with the original code in 1214aa841bc (reftable: add
blocksource, an abstraction for random access reads, 2021-10-07), so not
to be considered a regression for this release.

 reftable/blocksource.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/reftable/blocksource.c b/reftable/blocksource.c
index 0044eecd9aa..984bf07fc17 100644
--- a/reftable/blocksource.c
+++ b/reftable/blocksource.c
@@ -15,7 +15,8 @@ license that can be found in the LICENSE file or at
 
 static void strbuf_return_block(void *b, struct reftable_block *dest)
 {
-	memset(dest->data, 0xff, dest->len);
+	if (dest->data)
+		memset(dest->data, 0xff, dest->len);
 	reftable_free(dest->data);
 }
 
@@ -56,7 +57,8 @@ void block_source_from_strbuf(struct reftable_block_source *bs,
 
 static void malloc_return_block(void *b, struct reftable_block *dest)
 {
-	memset(dest->data, 0xff, dest->len);
+	if (dest->data)
+		memset(dest->data, 0xff, dest->len);
 	reftable_free(dest->data);
 }
 
@@ -85,7 +87,8 @@ static uint64_t file_size(void *b)
 
 static void file_return_block(void *b, struct reftable_block *dest)
 {
-	memset(dest->data, 0xff, dest->len);
+	if (dest->data)
+		memset(dest->data, 0xff, dest->len);
 	reftable_free(dest->data);
 }
 
-- 
2.36.0.rc2.283.gbef64175c85