[ruby/stringio] Fix each with multiple character string and chomp

Previously, this could result in an infinite loop.  Always update
the e pointer in this case, setting w when chomping so the chomped
data is not included in the output.

Fixes [Bug #18769]

https://github.com/ruby/stringio/commit/4bf64d5130

ext/stringio/stringio.c

@@ -1181,7 +1181,7 @@ strio_getline(struct getline_arg *arg, struct StringIO *ptr)
     const char *s, *e, *p;
     long n, limit = arg->limit;
     VALUE str = arg->rs;
-    int w = 0;
+    long w = 0;
     rb_encoding *enc = get_enc(ptr);
 
     if (ptr->pos >= (n = RSTRING_LEN(ptr->string))) {
@@ -1237,7 +1237,8 @@ strio_getline(struct getline_arg *arg, struct StringIO *ptr)
 	    if (e - s < 1024) {
 		for (p = s; p + n <= e; ++p) {
 		    if (MEMCMP(p, RSTRING_PTR(str), char, n) == 0) {
-			e = p + (arg->chomp ? 0 : n);
+			e = p + n;
+			w = (arg->chomp ? n : 0);
 			break;
 		    }
 		}

test/stringio/test_stringio.rb

@@ -584,6 +584,13 @@ class TestStringIO < Test::Unit::TestCase
     end
   end
 
+  def test_each_string_sep
+    f = StringIO.new('a||b||c')
+    assert_equal(["a||", "b||", "c"], f.each("||").to_a)
+    f.rewind
+    assert_equal(["a", "b", "c"], f.each("||", chomp: true).to_a)
+  end
+
   def test_each
     f = StringIO.new("foo\nbar\nbaz\n")
     assert_equal(["foo\n", "bar\n", "baz\n"], f.each.to_a)