WPA: Extra defense against PTK reinstalls in 4-way handshake

WPA: Extra defense against PTK reinstalls in 4-way handshake

Currently, reinstallations of the PTK are prevented by (1) assuring the
same TPTK is only set once as the PTK, and (2) that one particular PTK
is only installed once. This patch makes it more explicit that point (1)
is required to prevent key reinstallations. At the same time, this patch
hardens wpa_supplicant such that future changes do not accidentally
break this property.

Signed-off-by: Mathy Vanhoef <Mathy.Vanhoef@cs.kuleuven.be>