Fix KeyError crash with NBS binaries in testing

Traceback (most recent call last):
   File "/var/lib/britney/britney.py", line 3390, in <module>
     Britney().main()
   File "/var/lib/britney/britney.py", line 295, in __init__
     self.merge_binaries('testing', 'unstable', arch)
   File "/var/lib/britney/britney.py", line 785, in merge_binaries
     target_version = target_sources[value[SOURCE]][VERSION]
 KeyError: 'insighttoolkit4'

britney.py

@@ -781,7 +781,11 @@ class Britney(object):
             # that have built on this architecture.
             if value[SOURCE] not in oodsrcs:
                 source_version = source_sources[value[SOURCE]][VERSION]
-                target_version = target_sources[value[SOURCE]][VERSION]
+                try:
+                    target_version = target_sources[value[SOURCE]][VERSION]
+                except KeyError:
+                    self.__log("merge_binaries: pkg %s has no source, NBS?" % pkg)
+                    continue
                 if source_version != target_version:
                     current_arch = value[ARCHITECTURE]
                     built = False

tests/test_autopkgtest.py

@@ -1216,6 +1216,22 @@ class T(TestBase):
             {'lightgreen': [('old-version', '1'), ('new-version', '2')]}
         )
 
+    def test_nbs(self):
+        '''source-less binaries do not cause harm'''
+
+        # NBS in testing
+        self.data.add('liboldgreen0', False, add_src=False)
+        # NBS in unstable
+        self.data.add('liboldgreen1', True, add_src=False)
+        self.do_test(
+            [('libgreen1', {'Version': '2', 'Source': 'green'}, 'autopkgtest')],
+            {'green': (True, {'green 2': {'amd64': 'RUNNING-ALWAYSFAIL', 'i386': 'RUNNING-ALWAYSFAIL'},
+                              'lightgreen 1': {'amd64': 'RUNNING-ALWAYSFAIL', 'i386': 'RUNNING-ALWAYSFAIL'},
+                              'darkgreen 1': {'amd64': 'RUNNING-ALWAYSFAIL', 'i386': 'RUNNING-ALWAYSFAIL'},
+                             }),
+            },
+            {'green': [('old-version', '1'), ('new-version', '2')]})
+
     ################################################################
     # Tests for hint processing
     ################################################################