adb: fix public key authorization.

This was broken by commit e8b663fe, which removed the null-terminator
from the message sent. For some reason, adbd requires the key to be
null terminated, despite an explicit length being part of the message.

Bug: http://b/30971808
Change-Id: I9761298a57ed76ca2bc02b05490f98cdffbaa0c3
Test: manually tested, adb authorization dialog box shows up