buffer chunk after flushing buffer in write if paused

4c5a106 handled a convoluted case where there is a chunk in the buffer
AND  we're in a flowing state during a write call which caused out of
order writes.

The fix was to flush the buffer before emitting the new chunk, but it
didn't account for destinations pausing the stream after flushing part
of the buffer. This caused issues in npm/pacote/npm-registry-fetch.

That specific issue is demonstrated in everett1992/make-fetch-happen-tar-extract-error
and occurs when make-fetch-happen res.body is piped to a tar.extract
stream.

Fixes isaacs#27 npm/cli#3884 make-fetch-happen#63

index.js

@@ -165,7 +165,12 @@ module.exports = class Minipass extends Stream {
       // because we're mid-write, so that'd be bad.
       if (this[BUFFERLENGTH] !== 0)
         this[FLUSH](true)
-      this.emit('data', chunk)
+
+      // if we are still flowing after flushing the buffer we can emit the
+      // chunk otherwise we have to buffer it.
+      this.flowing
+        ? this.emit('data', chunk)
+        : this[BUFFERPUSH](chunk)
     } else
       this[BUFFERPUSH](chunk)