symbols: fix buffer overflow with illegal explicit group

symbols: fix buffer overflow with illegal explicit group

Trying ''./test/interactive -l us:5' causes us to crash.

The <layout>:<N> syntax says to put this layout at the N'th level.
However the code (inherited from xkbcomp) doesn't check that the group
is valid, and then happily indexes keyi->groups with it, which has a
static size of XKB_NUM_GROUPS (the SetExplicitGroup function assumes the
index is valid). So any value a user might put there > 4 makes nice
things happen.

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