logger.rb: Fix handling progname

Because progname was memoized with ||= a logger call that involved
outputting false would be nil. Example code:

  logger = Logger.new(STDOUT)
  logger.info(false)  # => nil

Perform an explicit nil check instead of ||= so that false will be output.

patched by Gavin Miller <gavingmiller@gmail.com> [Fix GH-1667]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@59380 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/logger.rb

@@ -457,7 +457,9 @@ def add(severity, message = nil, progname = nil)
     if @logdev.nil? or severity < @level
       return true
     end
-    progname ||= @progname
+    if progname.nil?
+      progname = @progname
+    end
     if message.nil?
       if block_given?
         message = yield

test/logger/test_logger.rb

@@ -235,6 +235,10 @@ def test_add
     log = log_add(logger, WARN, nil, "progname?")
     assert_equal("progname?\n", log.msg)
     assert_equal("my_progname", log.progname)
+    #
+    logger = Logger.new(nil)
+    log = log_add(logger, INFO, nil, false)
+    assert_equal("false\n", log.msg)
   end
 
   def test_level_log