Omit some highly unlikely conditional exits from deep-first search.

Omit some highly unlikely conditional exits from deep-first search.

With '--skeleton' re2c builds DFA skeleton and performs DFS in order to
estimate the size of data to be generated. It maintains size counter: as
soon as the counter reaches certain limit, DFS should stop.

Size counter is always checked when recursion returns.
Sometimes it is clear that size counter will overflow upon recursion
return (when arguments overflow already) and DFS can exit early (before
entering recursion).

This commit omits checks of some arguments (and correponding early exits
from DFS): first, path length is very unlikely to overflow (one has to
write/generate a regular expression with length of ~1Gb, in which case
skeleton generation won't be the worst problem); second, the number of
outgoing arcs in each vertex is also highly unlikely to exceed 1Gb limit.