dgit: --overwrite now no longer crashes if there is no $archive_hash

Ie there is nothing to overwrite (eg, when used with --new).  Instead,
it is simply ignored, as it is ignored in situations where the push is
fast forward.

Closes:#863576.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>

1 files changed, 2 insertions, 1 deletions

diff --git a/dgit b/dgit
index 3409e18..5a6b2b6 100755
--- a/dgit
+++ b/dgit
@@ -4301,7 +4301,8 @@ END
 	}
     }
 
-    if (defined $overwrite_version && !defined $maintviewhead) {
+    if (defined $overwrite_version && !defined $maintviewhead
+	&& $archive_hash) {
 	$dgithead = plain_overwrite_pseudomerge($clogp,
 						$dgithead,
 						$archive_hash);