Do not use VLA for big arrays

[mchalupa]

They lead to stack overflow, e.g.:

==30266==ERROR: AddressSanitizer: stack-overflow on address 0x7ffd6b2b44c8 (pc 0x5ddddca2af2b bp 0x7ffd71217730 sp 0x7ffd6b2b44d0 T0)
    #0 0x5ddddca2af2b in Automaton::top_LimAvg(weight_t*) const /home/marek/src/quak/src/Automaton.cpp:1708
    #1 0x5ddddca37708 in Automaton::compute_Top(value_function_t, weight_t*) const /home/marek/src/quak/src/Automaton.cpp:1969
    #2 0x5ddddca37a8c in Automaton::compute_Bottom(value_function_t, weight_t*) /home/marek/src/quak/src/Automaton.cpp:1979
    #3 0x5ddddca03226 in Automaton::getBottomValue(value_function_t) /home/marek/src/quak/src/Automaton.cpp:450
    #4 0x5ddddca20655 in Automaton::isConstant(value_function_t) /home/marek/src/quak/src/Automaton.cpp:1381
    #5 0x5ddddc9ef457 in main /home/marek/src/quak/experiments/constant.cpp:54
    #6 0x7549d1439c87  (/usr/lib/libc.so.6+0x25c87) (BuildId: 32a656aa5562eece8c59a585f5eacd6cf5e2307b)
    #7 0x7549d1439d4b in __libc_start_main (/usr/lib/libc.so.6+0x25d4b) (BuildId: 32a656aa5562eece8c59a585f5eacd6cf5e2307b)
    #8 0x5ddddc9ee904 in _start (/home/marek/src/quak/experiments/constant+0x1c0904) (BuildId: 318c9cffe36654346bd3e1167efae021c9e2e556)

SUMMARY: AddressSanitizer: stack-overflow /home/marek/src/quak/src/Automaton.cpp:1708 in Automaton::top_LimAvg(weight_t*) const