hudson.Launcher:ProcStarter::envs() may throw NPE

ProcStarter::envs() calls envs.clone(), but does not check envs variable for null. BTW, the variable may be null in the following case (see JENKINS-19506):

• There are several wrappers
• One of wrappers triggers remote call (Launcher::launch()) in setUp() or decorateLauncher() methods to setup variables (example: android-emulator) => it will make all other launchers to provide their environment variables
• One of other wrappers retrieves ProcStarter.envs() in setUp() or decorateLauncher()
• envs are empty for nested call => execution of other wrapper causes NPE inside core