Ensure sourceUrl is not null either

Author: Ruben Verdoes

LibGit2Sharp.Tests/CloneFixture.cs

@@ -234,5 +234,13 @@ public void CloningWithoutWorkdirPathThrows(string url)
         {
             Assert.Throws<ArgumentNullException>(() => Repository.Clone(url, null));
         }
+
+        [Fact]
+        public void CloningWithoutUrlThrows()
+        {
+            var scd = BuildSelfCleaningDirectory();
+
+            Assert.Throws<ArgumentNullException>(() => Repository.Clone(null, scd.DirectoryPath));
+        }
     }
 }

LibGit2Sharp/Repository.cs

@@ -538,6 +538,7 @@ public static string Discover(string startingPath)
         public static string Clone(string sourceUrl, string workdirPath,
             CloneOptions options = null)
         {
+            Ensure.ArgumentNotNull(sourceUrl, "sourceUrl");
             Ensure.ArgumentNotNull(workdirPath, "workdirPath");
 
             options = options ?? new CloneOptions();