Add some missing null-checks

Remove IsInitialized checks accidentally left in MessageParser
Simplify ExtensionCollection.CrossLink

csharp/src/Google.Protobuf/CodedOutputStream.cs

@@ -89,7 +89,7 @@ namespace Google.Protobuf
         private CodedOutputStream(byte[] buffer, int offset, int length)
         {
             this.output = null;
-            this.buffer = buffer;
+            this.buffer = ProtoPreconditions.CheckNotNull(buffer, nameof(buffer));
             this.position = offset;
             this.limit = offset + length;
             leaveOpen = true; // Simple way of avoiding trying to dispose of a null reference

csharp/src/Google.Protobuf/JsonFormatter.cs

@@ -133,7 +133,7 @@ namespace Google.Protobuf
         /// <param name="settings">The settings.</param>
         public JsonFormatter(Settings settings)
         {
-            this.settings = settings;
+            this.settings = ProtoPreconditions.CheckNotNull(settings, nameof(settings));
         }
 
         /// <summary>

csharp/src/Google.Protobuf/JsonParser.cs

@@ -110,7 +110,7 @@ namespace Google.Protobuf
         /// <param name="settings">The settings.</param>
         public JsonParser(Settings settings)
         {
-            this.settings = settings;
+            this.settings = ProtoPreconditions.CheckNotNull(settings, nameof(settings));
         }
 
         /// <summary>

csharp/src/Google.Protobuf/MessageParser.cs

@@ -72,7 +72,6 @@ namespace Google.Protobuf
         {
             IMessage message = factory();
             message.MergeFrom(data, DiscardUnknownFields, Extensions);
-            CheckMergedRequiredFields(message);
             return message;
         }
 
@@ -87,7 +86,6 @@ namespace Google.Protobuf
         {
             IMessage message = factory();
             message.MergeFrom(data, offset, length, DiscardUnknownFields, Extensions);
-            CheckMergedRequiredFields(message);
             return message;
         }
 
@@ -100,7 +98,6 @@ namespace Google.Protobuf
         {
             IMessage message = factory();
             message.MergeFrom(data, DiscardUnknownFields, Extensions);
-            CheckMergedRequiredFields(message);
             return message;
         }
 
@@ -113,7 +110,6 @@ namespace Google.Protobuf
         {
             IMessage message = factory();
             message.MergeFrom(input, DiscardUnknownFields, Extensions);
-            CheckMergedRequiredFields(message);
             return message;
         }
 
@@ -130,7 +126,6 @@ namespace Google.Protobuf
         {
             IMessage message = factory();
             message.MergeDelimitedFrom(input, DiscardUnknownFields, Extensions);
-            CheckMergedRequiredFields(message);
             return message;
         }
 
@@ -143,7 +138,6 @@ namespace Google.Protobuf
         {
             IMessage message = factory();
             MergeFrom(message, input);
-            CheckMergedRequiredFields(message);
             return message;
         }
 
@@ -176,12 +170,6 @@ namespace Google.Protobuf
             }
         }
 
-        internal static void CheckMergedRequiredFields(IMessage message)
-        {
-            if (!message.IsInitialized())
-                throw new InvalidOperationException("Parsed message does not contain all required fields");
-        }
-
         /// <summary>
         /// Creates a new message parser which optionally discards unknown fields when parsing.
         /// </summary>

csharp/src/Google.Protobuf/Reflection/ExtensionCollection.cs

@@ -107,13 +107,11 @@ namespace Google.Protobuf.Reflection
             {
                 descriptor.CrossLink();
 
-                IList<FieldDescriptor> _;
-                if (!declarationOrder.TryGetValue(descriptor.ExtendeeType, out _))
-                {
-                    declarationOrder.Add(descriptor.ExtendeeType, new List<FieldDescriptor>());
-                }
+                IList<FieldDescriptor> list;
+                if (!declarationOrder.TryGetValue(descriptor.ExtendeeType, out list))
+                    declarationOrder.Add(descriptor.ExtendeeType, list = new List<FieldDescriptor>());
 
-                declarationOrder[descriptor.ExtendeeType].Add(descriptor);
+                list.Add(descriptor);
             }
 
             extensionsByTypeInDeclarationOrder = declarationOrder