THRIFT-2359 TBufferedTransport doesn't clear it's buffer on a failed flush call

Patch: Chris Trotman
diff --git a/lib/php/lib/Thrift/Transport/TBufferedTransport.php b/lib/php/lib/Thrift/Transport/TBufferedTransport.php
index 0d3ad98..270c004 100644
--- a/lib/php/lib/Thrift/Transport/TBufferedTransport.php
+++ b/lib/php/lib/Thrift/Transport/TBufferedTransport.php
@@ -156,8 +156,13 @@
 
   public function flush() {
     if (TStringFuncFactory::create()->strlen($this->wBuf_) > 0) {
-      $this->transport_->write($this->wBuf_);
+      $out = $this->wBuf_;
+
+      // Note that we clear the internal wBuf_ prior to the underlying write
+      // to ensure we're in a sane state (i.e. internal buffer cleaned)
+      // if the underlying write throws up an exception
       $this->wBuf_ = '';
+      $this->transport_->write($out);
     }
     $this->transport_->flush();
   }