exec_util: re-use chunk buffer

Co-authored-by: Sutou Kouhei <kou@cozmixng.org>
Signed-off-by: Shizuo Fujita <fujita@clear-code.com>

Author: Watson1978

lib/fluent/compat/exec_util.rb

@@ -81,7 +81,8 @@ class JSONParser < Parser
         def call(io)
           parser = JSON::ResumableParser.new({})
           begin
-            while (chunk = io.readpartial(BYTES_TO_READ))
+            chunk = +"".b
+            while io.readpartial(BYTES_TO_READ, chunk)
               parser << chunk
               while parser.parse
                 @on_message.call(parser.value)