Fix match registers when position is larger than INT_MAX (#220)

## What

With `StringScanner.new(str, fixed_anchor: true)`, a scan position
beyond `INT_MAX` (a string longer than 2 GiB) makes
`StringScanner#getch` / `#get_byte` read out of bounds: a deterministic
`SIGSEGV` (and a bogus string returned when the address happens to be
mapped) instead of returning the byte.

## Reproducer

```ruby
require "strscan"

len = 2**31 + 64     # > INT_MAX
pos = 2**31 + 10

s  = "a" * len
sc = StringScanner.new(s, fixed_anchor: true)
sc.pos = pos
p sc.getch           # expected "a"; actual: SIGSEGV (exit 139)
```

On current master (`STRSCAN_VERSION 3.1.9`, built against Ruby 3.3.8):

```
control: fixed_anchor=false getch OK size=1 matched=1
trigger: calling getch with fixed_anchor=true ...
[BUG] Segmentation fault
strscan.so(extract_range)  strscan.c:170
strscan.so(strscan_getch)  strscan.c:1204
ruby exit code: 139
```

With the fix the same script completes normally and returns `"a"`.

## Root cause

`adjust_registers_to_matched()` stored the (64-bit) scan positions
through
`onig_region_set(region, 0, (int)p->prev, (int)p->curr)` — the `int`
parameters silently narrow the
positions. For `pos > INT_MAX` the stored `beg[0]` becomes negative;
`extract_range()` /
`extract_beg_len()` only checked the upper bound, so `S_PBEG(p) + beg_i`
is computed before the
string and `rb_str_new()` reads out of bounds.

## Changes

- store the match registers directly (`OnigPosition` values, no
narrowing).

## Tests

- Reproducer: crashes before the change / returns `"a"` after.
- `test/strscan/test_stringscanner.rb`: no new failures vs master (only
the pre-existing
  standalone-harness issues).

## Notes

This was also reported through HackerOne (`#4061020`), where Hiroshi
SHIBATA closed it as
Informative (the effect is a crash that requires an
application-controlled buffer larger than
2 GiB, rather than a security vulnerability) and asked to send it here
as a pull request.

Co-authored-by: Gkasgd <7713860+Gkasgd@users.noreply.github.com>

Author: Gkasgd

ext/strscan/strscan.c

@@ -1183,11 +1183,16 @@ static void
 adjust_registers_to_matched(struct strscanner *p)
 {
     onig_region_clear(&(p->regs));
+    if (onig_region_resize(&(p->regs), 1) != 0) return;
     if (p->fixed_anchor_p) {
-        onig_region_set(&(p->regs), 0, (int)p->prev, (int)p->curr);
+        /* Store absolute positions without narrowing them to int: scan
+         * positions can exceed INT_MAX (64-bit builds). */
+        p->regs.beg[0] = p->prev;
+        p->regs.end[0] = p->curr;
     }
     else {
-        onig_region_set(&(p->regs), 0, 0, (int)(p->curr - p->prev));
+        p->regs.beg[0] = 0;
+        p->regs.end[0] = p->curr - p->prev;
     }
 }