Fix buildorder recursion when there are no dependencies

This is a major problem since the AUR returns an error when the deplist
is empty.

Fixes #106.

src/auracle/auracle.cc

@@ -275,6 +275,10 @@ void Auracle::IteratePackages(std::vector<std::string> args,
               }
             }
 
+            if (alldeps.empty()) {
+              return 0;
+            }
+
             IteratePackages(std::move(alldeps), state);
           }
         }

tests/fakeaur/db/info/mingw-w64-environment

@@ -0,0 +1 @@
+{"resultcount":1,"results":[{"Description":"Script providing common environment variables and functions for MinGW (mingw-w64)","FirstSubmitted":1575319559,"ID":1504025,"Keywords":[],"LastModified":1720940851,"License":["BSD"],"Maintainer":"xantares","Name":"mingw-w64-environment","NumVotes":9,"OutOfDate":null,"PackageBase":"mingw-w64-environment","PackageBaseID":147146,"Popularity":0.490759,"Submitter":"xantares","URL":"http://fedoraproject.org/wiki/MinGW","URLPath":"/cgit/aur.git/snapshot/mingw-w64-environment.tar.gz","Version":"1-6"}],"type":"multiinfo","version":5}

tests/test_buildorder.py

@@ -122,6 +122,12 @@ def testDependencyCycle(self):
             'warning: found dependency cycle: [ python-fontpens -> python-fontparts -> python-fontpens ]',
             r.process.stderr.decode().strip().splitlines())
 
+    def testNoDependencies(self):
+        r = self.Auracle(['buildorder', 'mingw-w64-environment'])
+        self.assertEqual(0, r.process.returncode)
+        self.assertIn('TARGETAUR mingw-w64-environment mingw-w64-environment',
+                      r.process.stdout.decode().strip().splitlines())
+
 
 if __name__ == '__main__':
     auracle_test.main()