Fix a segfault in parsing OpenSSH private key files.
The initial test for a line ending with "PRIVATE KEY-----" failed to
take into account the possibility that the line might be shorter than
that. Fixed by introducing a new library function strendswith(), and
strstartswith() for good measure, and using that.
Thanks to Hanno Böck for spotting this, with the aid of AFL.
(cherry picked from commit
fa7b23ce9025daba08e86bb934fc430099792b9a)
Conflicts:
misc.c
misc.h
(cherry-picker's note: the conflicts were only due to other functions
introduced on trunk just next to the ones introduced by this commit)