[RFA] remote.c: Avoid multiple serial_close calls on baud rate error

[Andrew Cagney]

> One of my colleagues recently noticed the following:
> 
>     (gdb) set remotebaud 0x100000
>     (gdb) target remote /dev/ttyS0
>     warning: Invalid baud rate 1048576.  Maximum value is 460800.
>     /dev/ttyS0: Invalid argument.
>     (gdb) set remotebaud 230400
>     (gdb) target remote /dev/ttyS0
>     Segmentation fault
> 
> The reason for this SEGV is that remote.c was closing ``remote_desc''
> twice.  On the second attempted close, it was accessing some data
> structures through some already freed (and probably even reallocated)
> memory.
> 
> The comment that I've added explains how the double close is avoided.
> 
> FWIW, I considered calling remote_close(), but decided against it
> since remote_desc can not be passed explicitly to this function. 
> Also, if the implementation of remote_close() were to change in some
> way, it may end up doing more (or less) than what's desired for
> handling the baud rate error.  Conversely, a hypothetical change in
> remote_close() may require that the error handling code be changed in
> a similar fashion, so the preferred path to fixing this problem isn't
> quite so clear cut.  Therefore, I'm willing to revise this patch to
> call remote_close() instead if that's deemed preferable.
> 
> With regard to the testcase above, it'd be nice if this could be added
> to the testsuite, but I can't think of a portable way of doing so.

Calling target_close() here wouldn't be right.  The target isn't yet 
open, the push call only occures further down.  This begs the question: 
why was open called twice?  I suspect unpush_target should only call 
target_close on open/pushed targets.

Anyway, this change is fine.  It makes the relevant code more robust.

Andrew