Avoid to crash on outputting log at the early stage config parser

By #3352, an early stage config parser is introduced to enable setting
log rotation by system config. While this stage, the global logger
doesn't exist yet, so that it causes crash when the config parser try to
output log. This commit avoid the crash by checking $log. Same logs will
be output at the later stage so that it's not needed in the early stage.

See also: fluent/fluentd-kubernetes-daemonset#583

Signed-off-by: Takuro Ashie <ashie@clear-code.com>

lib/fluent/config/v1_parser.rb

@@ -37,6 +37,7 @@ def initialize(strscan, include_basepath, fname, eval_context)
         super(strscan, eval_context)
         @include_basepath = include_basepath
         @fname = fname
+        @logger = defined?($log) ? $log : nil
       end
 
       def parse!
@@ -99,7 +100,7 @@ def parse_element(root_element, elem_name, attrs = {}, elems = [])
 
           elsif root_element && skip(/(\@include|include)#{SPACING}/)
             if !prev_match.start_with?('@')
-              $log.warn "'include' is deprecated. Use '@include' instead"
+              @logger.warn "'include' is deprecated. Use '@include' instead" if @logger
             end
             parse_include(attrs, elems)
 
@@ -123,7 +124,7 @@ def parse_element(root_element, elem_name, attrs = {}, elems = [])
                     parse_error! "'@' is the system reserved prefix. Don't use '@' prefix parameter in the configuration: #{k}"
                   else
                     # TODO: This is for backward compatibility. It will throw an error in the future.
-                    $log.warn "'@' is the system reserved prefix. It works in the nested configuration for now but it will be rejected: #{k}"
+                    @logger.warn "'@' is the system reserved prefix. It works in the nested configuration for now but it will be rejected: #{k}" if @logger
                   end
                 end