Fix IOSource#readline for @pending_buffer (#215)

## Why? Fixed a problem that `@pending_buffer` is not processed when `IOError` occurs in `@source.readline` although `@pending_buffer` exists when reading XML file.

Author: naitoh

lib/rexml/parsers/baseparser.rb

@@ -167,6 +167,7 @@ def initialize( source )
 @entity_expansion_count=0
 @entity_expansion_limit=Security.entity_expansion_limit
 @entity_expansion_text_limit=Security.entity_expansion_text_limit
+@source.ensure_buffer
 end
 
 defadd_listener(listener)

lib/rexml/source.rb

@@ -295,14 +295,19 @@ def current_line
 
 private
 defreadline(term=nil)
-str=@source.readline(term || @line_break)
 if@pending_buffer
+begin
+str=@source.readline(term || @line_break)
+rescueIOError
+end
 ifstr.nil?
 str=@pending_buffer
 else
 str=@pending_buffer + str
 end
 @pending_buffer=nil
+else
+str=@source.readline(term || @line_break)
 end
 returnnilifstr.nil?
 

test/parse/test_text.rb

@@ -4,6 +4,23 @@
 moduleREXMLTests
 classTestParseText < Test::Unit::TestCase
 classTestInvalid < self
+deftest_text_only
+exception=assert_raise(REXML::ParseException)do
+parser=REXML::Parsers::BaseParser.new('a')
+whileparser.has_next?
+parser.pull
+end
+end
+
+assert_equal(<<~DETAIL.chomp,exception.to_s)
+ Malformed XML: Content at the start of the document (got 'a')
+ Line: 1
+ Position: 1
+ Last 80 unconsumed characters:
+
+ DETAIL
+end
+
 deftest_before_root
 exception=assert_raise(REXML::ParseException)do
 parser=REXML::Parsers::BaseParser.new('b<a></a>')