dmesg: Check input length before calling strtol

dmesg: Check input length before calling strtol

The dmesg input can be a memory mapped file, yet strtol is used to
parse numbers (facility, level, and syslog timestamp).

Make sure that a terminating character exists to stop strtol before
reaching end.

While at it, actually check if terminating character is the one we
were looking for.

Signed-off-by: Tobias Stoeckmann <tobias@stoeckmann.org>