Limit bidi_find_bracket_pairs to MAX_ALLOCA

Limit bidi_find_bracket_pairs to MAX_ALLOCA

* src/bidi.c (MAX_BPA_STACK): Now a constant, not a macro.
Shrink it to allow for the two struct bidi_it objects in
the same frame.
(PUSH_BPA_STACK): Avoid integer overflow with enormous bidi cache.
(bidi_find_bracket_pairs): Use compile-time check instead of runtime.