MFV r336490:

Prevent installation of an all-zero TK.
This is also upline git commit 53bb18cc8b7a4da72e47e4b3752d0d2135cffb23.

Obtained from: https://w1.fi/security/2017-1/\

		rebased-v2.6-0004-Prevent-installation-\
		of-an-all-zero-TK.patch

X-MFC-with: r336203