[ruby/openssl] read: don't clear buffer when nothing can be read

To be consistent with regular Ruby IOs:

```ruby
r, _ = IO.pipe
buf = "garbage".b
r.read_nonblock(10, buf, exception: false) # => :wait_readable
p buf # => "garbage"
```

Ref: https://github.com/redis-rb/redis-client/commit/98b8944460a11f8508217bda71cfc10cb2190d4d

https://github.com/ruby/openssl/commit/08452993d6

1 files changed, 7 insertions, 2 deletions

diff --git a/test/openssl/test_pair.rb b/test/openssl/test_pair.rb
index b616883925..66e36a7ab4 100644
--- a/test/openssl/test_pair.rb
+++ b/test/openssl/test_pair.rb
@@ -250,12 +250,17 @@ module OpenSSL::TestPairM
 
       buf = +"garbage"
       assert_equal :wait_readable, s2.read_nonblock(100, buf, exception: false)
-      assert_equal "", buf
+      assert_equal "garbage", buf
 
       s1.close
       buf = +"garbage"
-      assert_equal nil, s2.read(100, buf)
+      assert_nil s2.read(100, buf)
       assert_equal "", buf
+
+      buf = +"garbage"
+      ret = s2.read(0, buf)
+      assert_same buf, ret
+      assert_equal "", ret
     }
   end