reflog_write: don't access free()'d memory

We get the oid of a reference, free the reference, then
convert the oid to a string. We need to convert the oid
before freeing the memory.

src/reflog.c

@@ -237,10 +237,10 @@ int git_reflog_write(git_reference *ref, const git_oid *oid_old,
 		return error;
 	}
 
-	git_reference_free(r);
-
 	git_oid_to_string(new, GIT_OID_HEXSZ+1, oid);
 
+	git_reference_free(r);
+
 	error = git_buf_join_n(&log_path, '/', 3,
 		ref->owner->path_repository, GIT_REFLOG_DIR, ref->name);
 	if (error < GIT_SUCCESS)