Fix panic on starting exec more than once

Issue was caused when exec is tarted, exits, then stated again.
In this case, `Close` is called twice, which closes a channel twice.

Changes execConfig.ExitCode to a pointer so we can test if the it has
been set or not.
This allows us to return early when the exec has already been run.

Signed-off-by: Brian Goff <cpuguy83@gmail.com>
Upstream-commit: 1a60a805bfabee729dbc833515cd0be439adb95b
Component: engine

components/engine/daemon/exec/exec.go

@@ -18,7 +18,7 @@ type Config struct {
 	*runconfig.StreamConfig
 	ID            string
 	Running       bool
-	ExitCode      int
+	ExitCode      *int
 	ProcessConfig *execdriver.ProcessConfig
 	OpenStdin     bool
 	OpenStderr    bool