cli_classic: prevent corruption of flash when stdout/stderr is closed

While it's not posixly-correct, it's possible that a user, script, or
application may attempt to start flashrom with stdout or stderr
closed.  It's possible that we'll get a file descriptor of 1 or 2 when
opening a flash device (such as Linux MTD), and flashrom will send
garbage debug logs to the flash:

  # bash -c "exec >&- flashrom ..."

  Observed corruption:
  43 40 45 42 45 44 00 00 00 00 00 00 01 00 00 00 |C@EBED..........|
  00 02 00 00 63 65 73 73 66 75 6c 6c 79 0a 46 6f |....cessfully.Fo|
  75 6e 64 20 50 72 6f 67 72 61 6d 6d 65 72 20 66 |und Programmer f|
  6c 61 73 68 20 63 68 69 70 20 22 4f 70 61 71 75 |lash chip "Opaqu|
  65 20 66 6c 61 73 68 20 63 68 69 70 22 20 28 38 |e flash chip" (8|
  31 39 32 20 6b 42 2c 20 50 72 6f 67 72 61 6d 6d |192 kB, Programm|
  65 72 2d 73 70 65 63 69 66 69 63 29 20 6d 61 70 |er-specific) map|
  70 65 64 20 61 74 20 70 68 79 73 69 63 61 6c 20 |ped at physical |
  61 64 64 72 65 73 73 20 30 78 30 30 30 30 30 30 |address 0x000000|
  30 30 2e 0a ff ff ff ff ff ff ff ff ff ff ff ff |00..............|
  ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff |................|
  ...

While for most applications, closing stdout or stderr would just lead
to obsure bugs, for flashrom, we should have extra safety guards, as
this could mean that we might be bricking a device instead.

Add a basic safety check.

Signed-off-by: Jack Rosenthal <jrosenth@chromium.org>
Change-Id: I751c9dd88ad1d30283b94bd2185b4f8f25569c8f
---
M cli_classic.c
1 file changed, 23 insertions(+), 1 deletion(-)