MFC r352894: Don't use uninitialized memory.

Don't use stack memory which is not initialized.
Thanks to Mark Wodrich for reporting this issue for the userland stack in
https://github.com/sctplab/usrsctp/issues/380
This issue was also found for usrsctp by OSS-fuzz in
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=17778