ectool: Fix uninitialized variable warning
ectool: Fix uninitialized variable warning

gcc 4.8.4 warns read_buf is not initialized. This patch seizes the
warning.

BUG=none
BRANCH=none
TEST=make BOARD=stm32l476g-eval

Change-Id: I07a8e01ea285e3462d6b45604350ffeab6cc00e3
Signed-off-by: Daisuke Nojiri <fd5f93af191bf7e8f73ea71cc3c0f66b41b1dd49@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/359002
Reviewed-by: Randall Spangler <62698fdbb84d1779579ee80c3f39fac22017e5bc@chromium.org>

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