Use of the sync command without a matching acmd lead to a stack corruption

[AlbanBedel]

If an auto file use the sync command without a matching acmd, for example because of some naive copy&paste, then FD_SET() is called with g_stdout set to -1. This corrupt the stack which might go unnoticed, but if glibc has fortify support it will abort right away.

Even it is not a valid command sequence I think ufb should issue a warning and just ignore the command instead of corrupting the stack.

[nxpfrankli]

it is another project.
https://github.com/NXPmicro/imx-uuc/blob/master/ufb.c

May be crash at.
FD_SET(g_stdout, &rfds);