Add shell checking to cmdmod since shell can be defined.

salt/modules/cmdmod.py

@@ -56,6 +56,10 @@ def _run(cmd,
     if not cwd:
         cwd = os.path.expanduser('~{0}'.format('' if not runas else runas))
 
+    if not os.path.isfile(shell) or not os.access(shell, os.X_OK):
+        msg = 'The shell {0} is not available'.format(shell)
+        raise CommandExecutionError(msg)
+
     # TODO: Figure out the proper way to do this in windows
     disable_runas = [
         'Windows',