fix: guard read_body against HTTP/2 reload crash in captcha verify state

[drlogout]

Summary

Fixes a runtime crash when a client with a captcha-type decision reloads the
challenge page over HTTP/2. Closes #63.

Problem

ngx.req.read_body() raises an error for HTTP/2 (and HTTP/3) requests without
a Content-Length header. The body-read inside csmod.Allow()'s captcha
verify-state handler is unguarded, so any HTTP/2 GET reload of the captcha
page crashes nginx with HTTP 500:

lua entry thread aborted: runtime error:
.../crowdsec.lua:NNN: http2 requests are not supported without content-length header
stack traceback:
[C]: in function 'read_body'
.../crowdsec.lua:NNN: in function 'Allow'

The get_body() helper already implements the correct guard; this PR applies
the same pattern to the second read_body call site.

Reproduction

1. Deploy crowdsec-nginx-bouncer with a captcha provider configured (e.g. Turnstile, hCaptcha)
2. Serve a site over HTTP/2 (default for HTTPS on modern nginx)
3. Force a captcha decision: cscli decisions add -i <your-ip> -t captcha -d 10m
4. Visit the site in a browser → captcha page renders (first request, state cache empty)
5. Reload the page → HTTP 500 + read_body error in nginx error log

Fix

Mirror the HTTP/2 protocol-version guard from get_body(). When the body is
unreadable, skip the read and fall through with an empty args table, which
correctly re-serves the captcha page. Captcha form submissions are POSTs
with Content-Length, so they're unaffected.

Compatibility

• HTTP/1.1: behavior unchanged
• HTTP/2 GET (e.g. reload of captcha page): no longer crashes; re-serves captcha
• HTTP/2 POST with Content-Length (captcha form submission): behavior unchanged
• HTTP/2 POST without Content-Length: body skipped (same as get_body behavior)

Tested

• Reproduced the crash on a production deployment of crowdsec-nginx-bouncer v1.1.6
• Applied the patch and verified:
• First captcha load renders normally
• Reload of the captcha page no longer crashes
• Captcha form submission validates and unblocks the IP as expected
• Verified no regression in HTTP/1.1 flow
  EOF
  )"