Don't calculate smoothbins for sources not in unstable

When building the excuses, don't calculate smoothbins if the source is not in
unstable. Doing so would result in a KeyError and a crash. This happens for
sources that are in testing and (testing-)proposed-updates, but not in
unstable.

Signed-off-by: Ivo De Decker <ivodd@debian.org>

britney.py

@@ -1295,12 +1295,14 @@ class Britney(object):
             # removing binary packages
             primary_source_suite = self.suite_info.primary_source_suite
             if not (ssrc and source_suite is not primary_source_suite):
-                # for every binary package produced by this source in testing for this architecture
+                smoothbins = set()
+                if src in primary_source_suite.sources:
                     _, _, smoothbins, _ = self._compute_groups(src,
                                                             primary_source_suite,
                                                             arch,
                                                             False)
 
+                # for every binary package produced by this source in testing for this architecture
                 for pkg_id in sorted(x for x in source_t.binaries if x.architecture == arch):
                     pkg = pkg_id.package_name
                     # if the package is architecture-independent, then ignore it