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]

string.c

@@ -3676,8 +3676,14 @@ rb_str_index_m(int argc, VALUE *argv, VALUE str)
 	pos = str_offset(RSTRING_PTR(str), RSTRING_END(str), pos,
 			 rb_enc_check(str, sub), single_byte_optimizable(str));
 
-	pos = rb_reg_search(sub, str, pos, 0);
-	pos = rb_str_sublen(str, pos);
+	if (rb_reg_search(sub, str, pos, 0) < 0) {
+            return Qnil;
+        } else {
+            VALUE match = rb_backref_get();
+            struct re_registers *regs = RMATCH_REGS(match);
+            pos = rb_str_sublen(str, BEG(0));
+            return LONG2NUM(pos);
+        }
     }
     else {
         StringValue(sub);
@@ -3827,9 +3833,12 @@ rb_str_rindex_m(int argc, VALUE *argv, VALUE str)
 	pos = str_offset(RSTRING_PTR(str), RSTRING_END(str), pos,
 			 enc, single_byte_optimizable(str));
 
-	pos = rb_reg_search(sub, str, pos, 1);
-	pos = rb_str_sublen(str, pos);
-	if (pos >= 0) return LONG2NUM(pos);
+	if (rb_reg_search(sub, str, pos, 1) >= 0) {
+            VALUE match = rb_backref_get();
+            struct re_registers *regs = RMATCH_REGS(match);
+            pos = rb_str_sublen(str, BEG(0));
+            return LONG2NUM(pos);
+        }
     }
     else {
         StringValue(sub);

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