Fix possible crash in --list-debian-substvars

Fixes: #228



Signed-off-by: Mattia Rizzolo <mattia@debian.org>

diffoscope/main.py

@@ -586,7 +586,8 @@ class ListDebianSubstvarsAction(argparse._StoreTrueAction):
             try:
                 pkg = EXTERNAL_TOOLS[x]["debian"]
             except KeyError:
-                pass
+                continue
+
             packages.add(pkg)
             if x not in HUGE_TOOLS:
                 packages_minimal.add(pkg)