Correctly resize ByteBuffers (#150)

Motivation:

In two different places in this module we "reserve" space by
moving the writer index forward. This isn't the correct way to
do that, and it's a bit of a needless performance
optimization. Instead, just write a zero in that location and
come back to it.

Modifications:

- Replace unmotivated writer index move with a write.
- Add tests.

Result:

Fewer crashes

Sources/NIOSSH/ByteBuffer+SSH.swift

@@ -205,7 +205,7 @@ extension ByteBuffer {
     mutating func writeCompositeSSHString(_ compositeFunction: (inout ByteBuffer) throws -> Int) rethrows -> Int {
         // Reserve 4 bytes for the length.
         let originalWriterIndex = self.writerIndex
-        self.moveWriterIndex(forwardBy: 4)
+        self.writeInteger(UInt32(0))
 
         var writtenLength: Int
         do {

Sources/NIOSSH/SSHPacketSerializer.swift

@@ -73,7 +73,7 @@ extension ByteBuffer {
         ///   byte[m]   mac (Message Authentication Code - MAC); m = mac_length
 
         /// payload
-        self.moveWriterIndex(forwardBy: 5)
+        self.writeMultipleIntegers(UInt32(0), UInt8(0))
         let messageLength = self.writeSSHMessage(message)
 
         // Depending on on whether packet length is encrypted, padding should reflect that

Tests/NIOSSHTests/ByteBuffer+SSHTests.swift

@@ -361,6 +361,33 @@ final class ByteBufferSSHTests: XCTestCase {
 
         XCTAssertNoThrow(XCTAssertNotNil(try buffer.readSSHHostKey()))
     }
+
+    func testCompositeStringDoesTheRightThingWithBB() throws {
+        var buffer = ByteBuffer()
+        XCTAssertEqual(buffer.capacity, 0)
+
+        buffer.writeCompositeSSHString {
+            $0.writeInteger(UInt64(9))
+        }
+        let writtenBytes = buffer.readBytes(length: buffer.readableBytes)
+        XCTAssertEqual(
+            writtenBytes,
+            [0, 0, 0, 8, 0, 0, 0, 0, 0, 0, 0, 9]
+        )
+    }
+
+    func testWriteSSHPacketDoesTheRightThingWithBB() throws {
+        var buffer = ByteBuffer()
+        XCTAssertEqual(buffer.capacity, 0)
+
+        buffer.writeSSHPacket(message: .version("Tests_v1.0"), lengthEncrypted: false, blockSize: 8)
+
+        let writtenBytes = buffer.readBytes(length: 5)
+        XCTAssertEqual(
+            writtenBytes,
+            [0, 0, 0, 24, 11]
+        )
+    }
 }
 
 extension ByteBuffer {