Guard against null socket in nextWrite

When a connection closes, `closed()` sets `socket = null` (after rejecting
any in-flight query with CONNECTION_CLOSED). A write smaller than 1024 bytes
schedules `setImmediate(nextWrite)`; if the connection closes before that
callback runs, `nextWrite` dereferences a null socket and throws an
uncatchable `TypeError: Cannot read properties of null (reading 'write')`,
crashing the process.

Add an early return when the socket is already gone. At that point the
query has already been rejected through the normal close path, so the
buffered write is moot — skipping it is safe and simply avoids the crash.

Fixes #1066, #1154

Author: geordie-danim

cf/src/connection.js

@@ -254,6 +254,8 @@ function Connection(options, queues = {}, { onopen = noop, onend = noop, onclose
   }
 
   function nextWrite(fn) {
+    if (!socket)
+      return false
     const x = socket.write(chunk, fn)
     nextWriteTimer !== null && clearImmediate(nextWriteTimer)
     chunk = nextWriteTimer = null

cjs/src/connection.js

@@ -252,6 +252,8 @@ function Connection(options, queues = {}, { onopen = noop, onend = noop, onclose
   }
 
   function nextWrite(fn) {
+    if (!socket)
+      return false
     const x = socket.write(chunk, fn)
     nextWriteTimer !== null && clearImmediate(nextWriteTimer)
     chunk = nextWriteTimer = null

deno/src/connection.js

@@ -255,6 +255,8 @@ function Connection(options, queues = {}, { onopen = noop, onend = noop, onclose
   }
 
   function nextWrite(fn) {
+    if (!socket)
+      return false
     const x = socket.write(chunk, fn)
     nextWriteTimer !== null && clearImmediate(nextWriteTimer)
     chunk = nextWriteTimer = null

src/connection.js

@@ -252,6 +252,8 @@ function Connection(options, queues = {}, { onopen = noop, onend = noop, onclose
   }
 
   function nextWrite(fn) {
+    if (!socket)
+      return false
     const x = socket.write(chunk, fn)
     nextWriteTimer !== null && clearImmediate(nextWriteTimer)
     chunk = nextWriteTimer = null