spawn: Fix crash when called with an empty table.

The spawn code didn't properly handle the case where there
is an empty command stream. In that case, no error is
reported as there is simply nothing to do. The error message
was probed and this caused an invalid read and crash.

Fix #1033

spawn.c

@@ -414,8 +414,12 @@ luaA_spawn(lua_State *L)
     if(!argv || !argv[0])
     {
         g_strfreev(argv);
-        luaA_warn(L, "spawn: parse error: %s", error->message);
-        g_error_free(error);
+        if (error) {
+            luaA_warn(L, "spawn: parse error: %s", error->message);
+            g_error_free(error);
+        }
+        else
+            luaA_warn(L, "spawn: There is nothing to execute");
         return 1;
     }