[ruby/stringio] [Bug #19389] Fix chomping with longer separator

https://github.com/ruby/stringio/commit/eb322a9716

ext/stringio/stringio.c

@@ -1340,8 +1340,9 @@ strio_getline(struct getline_arg *arg, struct StringIO *ptr)
 	str = strio_substr(ptr, ptr->pos, e - s - w, enc);
     }
     else {
-	if (n < e - s) {
-	    if (e - s < 1024) {
+	if (n < e - s + arg->chomp) {
+	    /* unless chomping, RS at the end does not matter */
+	    if (e - s < 1024 || n == e - s) {
 		for (p = s; p + n <= e; ++p) {
 		    if (MEMCMP(p, RSTRING_PTR(str), char, n) == 0) {
 			e = p + n;

test/stringio/test_stringio.rb

@@ -99,6 +99,8 @@ class TestStringIO < Test::Unit::TestCase
     assert_equal("def\n", stringio.gets("", chomp: true))
 
     assert_string("", Encoding::UTF_8, StringIO.new("\n").gets(chomp: true))
+
+    assert_equal("", StringIO.new("ab").gets("ab", chomp: true))
   end
 
   def test_gets_chomp_eol