packer/plugin: fix crash case, nil function call [GH-2098]

CHANGELOG.md

@@ -12,6 +12,7 @@ IMPROVEMENTS:
 
 BUG FIXES:
 
+  * core: Fix potential panic for post-processor plugin exits [GH-2098]
   * builder/amazon: Remove deprecated ec2-upload-bundle paramger [GH-1931]
   * builder/amazon: Retry finding created instance for eventual
       consistency. [GH-2129]

packer/plugin/post_processor.go

@@ -29,7 +29,7 @@ func (c *cmdPostProcessor) PostProcess(ui packer.Ui, a packer.Artifact) (packer.
 }
 
 func (c *cmdPostProcessor) checkExit(p interface{}, cb func()) {
-	if c.client.Exited() {
+	if c.client.Exited() && cb != nil {
 		cb()
 	} else if p != nil && !Killed {
 		log.Panic(p)