i2ctrace: add safety check
i2ctrace: add safety check

Prevent NULL dereference coming from get_i2c_port

BUG=b:64477774
BRANCH=none
TEST=none

Signed-off-by: Boris Mittelberg <46897f4fb5cad4b1d5146ad1fd460d7f4825bbf7@google.com>
Change-Id: Ib430832e135a8f48cd3773acd2ce9894444f7efe
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/4019346
Reviewed-by: caveh jalali <08e24a66aefda7a55ca7d73f8903e0fb16eec763@chromium.org>
Code-Coverage: Zoss <345cf107e82c70959d46f59d98bddc98ce458aff@prod.google.com>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec