fix NPE in Path.Combine if binPath isn't set

in some special environments (e.g. game engines) AppDomain BaseDirectory isn't set.
Workaround: new Configuration().SetProperty(
                "nhibernate-logger",
                typeof(Log4NetLoggerFactory).AssemblyQualifiedName
            )

Author: childnode

src/NHibernate/Logging.cs

@@ -85,7 +85,7 @@ private static string GetNhibernateLoggerClass()
 				string baseDir = AppDomain.CurrentDomain.BaseDirectory;
 				string relativeSearchPath = AppDomain.CurrentDomain.RelativeSearchPath;
 				string binPath = relativeSearchPath == null ? baseDir : Path.Combine(baseDir, relativeSearchPath);
-				var log4NetDllPath = Path.Combine(binPath, "log4net.dll");
+				string log4NetDllPath = binPath == null ? "log4net.dll" : Path.Combine(binPath, "log4net.dll");
 
 				if (File.Exists(log4NetDllPath))
 				{
@@ -456,4 +456,4 @@ public void WarnFormat(string format, params object[] args)
 				WarnFormatDelegate(logger, format, args);
 		}
 	}
-}
+}