fix chomping newline only line

* string.c (chomp_newline): fix chomping newline only line.
  rb_enc_prev_char return NULL if no previous character and must
  not call rb_enc_ascget on it.  a patch by Ary Borenszweig
  <asterite AT gmail.com> at [ruby-core:78666].  [Bug #13037]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@57088 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

2 files changed, 13 insertions, 1 deletions

diff --git a/string.c b/string.c
index 8515773a84..fd17355346 100644
--- a/string.c
+++ b/string.c
@@ -7409,7 +7409,7 @@ chomp_newline(const char *p, const char *e, rb_encoding *enc)
     if (rb_enc_is_newline(prev, e, enc)) {
 	e = prev;
 	prev = rb_enc_prev_char(p, e, e, enc);
-	if (rb_enc_ascget(prev, e, NULL, enc) == '\r')
+	if (prev && rb_enc_ascget(prev, e, NULL, enc) == '\r')
 	    e = prev;
     }
     return e;
diff --git a/test/ruby/test_string.rb b/test/ruby/test_string.rb
index 39477be220..d2fbd00a1d 100644
--- a/test/ruby/test_string.rb
+++ b/test/ruby/test_string.rb
@@ -843,6 +843,18 @@ CODE
 
     assert_equal "hello", S("hello\nworld").each_line(chomp: true).next
     assert_equal "hello\nworld", S("hello\nworld").each_line(nil, chomp: true).next
+
+    res = []
+    S("").each_line(chomp: true) {|x| res << x}
+    assert_equal([], res)
+
+    res = []
+    S("\n").each_line(chomp: true) {|x| res << x}
+    assert_equal([S("")], res)
+
+    res = []
+    S("\r\n").each_line(chomp: true) {|x| res << x}
+    assert_equal([S("")], res)
   end
 
   def test_lines