rgw/sfs: Fix s3tests_boto3.functional.test_s3.test_post_object_* bug

`mtime` parameter was logged but that's an output variable and might be
`nullptr`.

Avoid logging it and check if it's `nullptr` before setting the value.

Fixes: aquarist-labs/s3gw-tools#191
Signed-off-by: Xavi Garcia <xavi.garcia@suse.com>

src/rgw/driver/sfs/CHANGELOG.md

@@ -7,6 +7,13 @@ The format is based on
 and this project adheres to
 [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## Unreleased
+
+### Fixed
+
+- Fixed segfault when SFSAtomicWriter::complete is called whith mtime output
+  variable set to nullptr
+
 ## [0.7.0] - 2022-10-20
 
 ### Fixed

src/rgw/driver/sfs/writer.cc

@@ -98,7 +98,7 @@ int SFSAtomicWriter::complete(
     optional_yield y
 ) {
   lsfs_dout(dpp, 10) << "accounted_size: " << accounted_size
-                     << ", etag: " << etag << ", mtime: " << to_iso_8601(*mtime)
+                     << ", etag: " << etag
                      << ", set_mtime: " << to_iso_8601(set_mtime)
                      << ", attrs: " << attrs
                      << ", delete_at: " << to_iso_8601(delete_at)
@@ -116,7 +116,9 @@ int SFSAtomicWriter::complete(
   meta.attrs = attrs;
   bucketref->finish(dpp, obj.get_name());
 
-  *mtime = meta.mtime;
+  if (mtime != nullptr) {
+    *mtime = meta.mtime;
+  }
   objref->metadata_finish(store);
   return 0;
 }