agetty: Fix a use of uninitialised memory in an agetty error path.

agetty: Fix a use of uninitialised memory in an agetty error path.

get_logname() assumes that when it calls read() it initializes c and
errno, which isn't always true if we hit a whitelisted error or end of
file. This occasionally shows up as agetty going into an infinite
loop. Fix it by just delaying ten seconds and exiting when things go
wrong, similarly to the behavior after a non-whitelisted error.

[kzak@redhat.com: - interpret readres == 0 as c = 0
                  - ignore speed configurations for VCONSOLE]

Signed-off-by: Steven Smith <sos22@srcf.ucam.org>
Signed-off-by: Karel Zak <kzak@redhat.com>