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>