invalid base shouldn't silently fail

Revert 44a5809224e819bf8bcca4ae9a8c5ea5bf4cefd7 and check range of
`base` with `mrb_assert`.
It actually can only be either 2, 8, 10 or 16 (and never 0), unless
mruby sources are modified. But I don't see a reason to impose further
limits.
Merge pull request #2455 from cremno/readint_mrb_int-check-base

invalid base shouldn't silently fail