rb_str_{index,rindex}_m: Handle /\K/ in pattern

When the pattern Regexp given to String#index and String#rindex
contain a /\K/ (lookbehind) operator, these methods return the
position where the beginning of the lookbehind pattern matches, while
they are expected to return the position where the \K matches.

```
# without patch
"abcdbce".index(/b\Kc/)  # => 1
"abcdbce".rindex(/b\Kc/)  # => 4
```

This patch fixes this problem by using BEG(0) instead of the return
value of rb_reg_search.

```
# with patch
"abcdbce".index(/b\Kc/)  # => 2
"abcdbce".rindex(/b\Kc/)  # => 5
```

Fixes [Bug #17118]

1 files changed, 4 insertions, 0 deletions

diff --git a/test/ruby/test_string.rb b/test/ruby/test_string.rb
index 308ba80373..489eee33dc 100644
--- a/test/ruby/test_string.rb
+++ b/test/ruby/test_string.rb
@@ -1353,6 +1353,8 @@ CODE
 
     assert_nil("foo".index(//, -100))
     assert_nil($~)
+
+    assert_equal(2, S("abcdbce").index(/b\Kc/))
   end
 
   def test_insert
@@ -1525,6 +1527,8 @@ CODE
 
     assert_equal(3, "foo".rindex(//))
     assert_equal([3, 3], $~.offset(0))
+
+    assert_equal(5, S("abcdbce").rindex(/b\Kc/))
   end
 
   def test_rjust