crash-reporter: Replace NULL with nullptr. BUG=None TEST=`FEATURES=test emerge-$BOARD crash-reporter` Change-Id: If0804613ee0385752d01f7bbe01902ffef53bd94 Reviewed-on: https://chromium-review.googlesource.com/218870 Reviewed-by: Alex Vakulenko <[email protected]> Commit-Queue: Ben Chan <[email protected]> Tested-by: Ben Chan <[email protected]>