Don't crash when no valid patches are found

Error out when we don't find any patches in an mbox when trying to diff
series.

Reported-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Konstantin Ryabitsev <konstantin@linuxfoundation.org>

1 files changed, 3 insertions, 0 deletions

diff --git a/b4/diff.py b/b4/diff.py
index 7e9125b..4d30754 100644
--- a/b4/diff.py
+++ b/b4/diff.py
@@ -102,6 +102,9 @@ def diff_mboxes(cmdargs):
         lmbx = b4.LoreMailbox()
         for key, msg in mbx.items():
             lmbx.add_message(msg)
+        if len(lmbx.series) < 1:
+            logger.critical('No valid patches found in %s', mboxfile)
+            sys.exit(1)
         if len(lmbx.series) > 1:
             logger.critical('More than one series version in %s, will use latest', mboxfile)