symbols: avoid possible access-out-of-bound due to explicit_group

symbols: avoid possible access-out-of-bound due to explicit_group

The code that handles group name statements currently does this:
    info->group_names[grp - 1 + info->explicit_group] = name;
Other than the fact that this addition makes no sense, it actually can
reach out of the bounds of the array (which is of size XKB_NUM_GROUPS)
in the (non-realistic) case where (grp - 1) is not 0 (i.e. the statement
is not name[Group1] = "foo").

We also change explicit_group to be XKB_LAYOUT_INVALID if not set
otherwise, instead of initializing it to 0; this is clearer and if
someone happens to write 'us:1' for some reason, it will discard the
other groups in the file as it should.

This entire explicit_group thing was clearly bolted on as an
afterthought.

Signed-off-by: Ran Benita <ran234@gmail.com>