get_linux_config.sh: sets THIS_MINOR to 0 if not valid.

Sets THIS_MINOR to 0 if not valid, ie when processing a Linux config file
which does not have a minor number extension;
ex: "<name>-linux-2.6.<major>.config"
Otherwise bash is raising an error/exception (line 59) and script can not
complete.

Signed-off-by: Julien Boibessot <julien.boibessot@armadeus.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>