When hardware write protect is applied, flashrom crashed and
generate coredump. spi_disable_blockprotect_generic() calls
flash->chip->printlock() method when disable was failed,
but this method is optional, can be NULL depends on type of
flashrom chip. NULL pointer check before call is added to
avoid crash.

BRANCH=none
BUG=b:129083894
TEST=Run on Mistral P2
(On CR50 console, run "wp disable")
flashrom --wp-range 0 0x400000
flashrom --wp-enable
(On CR50 console, run "wp enable")
flashrom -r /tmp/test.bin
Verify "Block protection could not be disabled!" is shown,
but flash read completes.
Signed-off-by: Yuji Sasaki <sasakiy@chromium.org>

Change-Id: I81094ab5f16a85871fc9869a2e285eddbbbdec4e
Reviewed-on: https://chromium-review.googlesource.com/1535140


Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Tested-by: Edward O'Callaghan <quasisec@chromium.org>
Reviewed-by: Stefan Reinauer <reinauer@google.com>
Revie...