Fix buffer overflow by only using snprintf

[oleg-vinted]

Pulling from vinted#17:

This piece of code contains a buffer overflow issue because it uses a combination of memcpy and snprintf calls that don't take the remaining buffer capacity into account.

I've used the approach used on line 97 which uses a single snprintf call to make sure we don't overrun the buffer.

Without the patch, CI fails with the following crash:

cp vendor/libmemcached-0.32/tests/udp.c tmp/x86_64-linux/stage/vendor/libmemcached-0.32/tests/udp.c
install -c tmp/x86_64-linux/rlibmemcached/3.0.7/rlibmemcached.so lib/rlibmemcached.so
cp tmp/x86_64-linux/rlibmemcached/3.0.7/rlibmemcached.so tmp/x86_64-linux/stage/lib/rlibmemcached.so
/home/runner/work/memcached/memcached/lib/memcached.rb:26: warning: already initialized constant Memcached::VERSION
/home/runner/work/memcached/memcached/lib/memcached/version.rb:2: warning: previous definition of VERSION was here
memcached(2403): Operation not permitted
/home/runner/work/memcached/memcached/test/unit/rails_test.rb:9: warning: constant Memcached::Rails is deprecated
/home/runner/work/memcached/memcached/test/unit/rails_test.rb:9: warning: constant Memcached::Rails is deprecated
Loaded suite /home/runner/work/memcached/memcached/vendor/bundle/ruby/3.0.0/gems/rake-13.0.3/lib/rake/rake_test_loader
Started
*** buffer overflow detected ***: terminated
Aborted (core dumped)
rake aborted!
Command failed with status (134)
/home/runner/work/memcached/memcached/vendor/bundle/ruby/3.0.0/gems/rake-13.0.3/exe/rake:27:in `<top (required)>'
/opt/hostedtoolcache/Ruby/3.0.7/x64/bin/bundle:23:in `load'
/opt/hostedtoolcache/Ruby/3.0.7/x64/bin/bundle:23:in `<main>'
Tasks: TOP => default => test
(See full trace by running task with --trace)

[oleg-vinted]

Hello, this PR has been up for a bit, do you accept contributions to this repo? We have our own fork (https://github.com/vinted/memcached/tree/1-0-stable) and I'd like prevent our fork from diverging too much from yours.

(There's one more pending PR: #33.)

Tagging the author of the last commit: @peterzhu2118

[oleg-vinted]

Tagging more maintainers: @casperisfine

[casperisfine]

do you accept contributions to this repo?

I guess, but we're no longer using it (or about to stop, can't quite remember).

[casperisfine]

Code looks fine, but the test suite is hanging on my machine (not just your branch), so it doesn't help with confidently making change to this repo.

This gem is such a tire fire...