wpa: Fix EAP/PEAP MSCHAPv2 authentication SIGSEGV

25ecdc7d52770caf1c9b44b5ec11f468f6b636f3 (MFCed by
5654815fd3604e024eefdcb34904d3a7c883e8c5) introduced a link error
causing a SIGSEGV when using EAP/PEAP MSCHAPv2 authentication. It was
subsequently addressed by c1d255d3ffdbe447de3ab875bf4e7d7accc5bfc5,
discovered by build time link errors not experienced during testing of
25ecdc7d52770caf1c9b44b5ec11f468f6b636f3. This commit MFCs a portion
of c1d255d3ffdbe447de3ab875bf4e7d7accc5bfc5 addressing only the
SIGSEGV. The rest of c1d255d3ffdbe447de3ab875bf4e7d7accc5bfc5 will
be MFCed in November 2021.

This is a direct commit to stable/12.

PR: 258527
Reported by: Marek Zarychta <zarychtam@plan-b.pwste.edu.pl>
Tested by: Marek Zarychta <zarychtam@plan-b.pwste.edu.pl>