Fix snapshot crashes

We did not handle multiple components properly, add a contrib
component to the test case.

Author: julian-klode

apt-pkg/deb/debmetaindex.cc

@@ -1158,6 +1158,8 @@ class APT_HIDDEN debSLTypeDebian : public pkgSourceList::Type		/*{{{*/
       std::transform(Options.begin(), Options.end(), std::back_inserter(ret),
 		     [](auto &&O) { return O.first; });
       std::sort(ret.begin(), ret.end());
+      auto r = std::remove(ret.begin(), ret.end(), "SHADOWED");
+      ret.erase(r, ret.end());
       return ret;
    }
 
@@ -1315,7 +1317,7 @@ class APT_HIDDEN debSLTypeDebian : public pkgSourceList::Type		/*{{{*/
 	 std::string filename;
 
 	 // The Release file and config based on that should be the ultimate source of truth.
-	 if (ReleaseFileName(Deb, filename))
+	 if (Deb && ReleaseFileName(Deb, filename))
 	 {
 	    auto OldDeb = dynamic_cast<debReleaseIndex *>(Deb->UnloadedClone());
 	    if (not OldDeb->Load(filename, nullptr))