String#b: Don't depend on dependent string

Registering a string that depend on a dependent string as fstring
can lead to use-after-free. See c06ddfe and 3f95620 for details.

The following script triggers use-after-free on trunk, 2.4.6, 2.5.5
and 2.6.3. Credits to @wanabe for using eval as a cross-version way
of registering a fstring.

```ruby
a = ('j' * 24).b.b
eval('', binding, a)

p a
4.times { GC.start }
p a
```

 - string.c (str_replace_shared_without_enc): when given a
   dependent string, depend on the root of the dependent
   string.

[Bug #15934]

string.c

@@ -1161,19 +1161,26 @@ str_replace_shared_without_enc(VALUE str2, VALUE str)
 	TERM_FILL(ptr2+len, termlen);
     }
     else {
-	str = rb_str_new_frozen(str);
+        VALUE root;
+        if (STR_SHARED_P(str)) {
+            root = RSTRING(str)->as.heap.aux.shared;
+            RSTRING_GETMEM(str, ptr, len);
+        }
+        else {
+            root = rb_str_new_frozen(str);
+            RSTRING_GETMEM(root, ptr, len);
+        }
         if (!STR_EMBED_P(str2) && !FL_TEST_RAW(str2, STR_SHARED|STR_NOFREE)) {
             /* TODO: check if str2 is a shared root */
             char *ptr2 = STR_HEAP_PTR(str2);
-            if (STR_HEAP_PTR(str) != ptr2) {
+            if (ptr2 != ptr) {
                 ruby_sized_xfree(ptr2, STR_HEAP_SIZE(str2));
             }
         }
 	FL_SET(str2, STR_NOEMBED);
-	RSTRING_GETMEM(str, ptr, len);
 	RSTRING(str2)->as.heap.len = len;
 	RSTRING(str2)->as.heap.ptr = ptr;
-	STR_SET_SHARED(str2, str);
+	STR_SET_SHARED(str2, root);
     }
     return str2;
 }

test/ruby/test_string.rb

@@ -2983,6 +2983,14 @@ CODE
     assert_equal(('a' * 24), a, '[Bug #15792]')
   end
 
+  def test_nesting_shared_b
+    a = ('j' * 24).b.b
+    eval('', binding, a)
+    assert_equal(('j' * 24), a)
+    4.times { GC.start }
+    assert_equal(('j' * 24), a, '[Bug #15934]')
+  end
+
   def test_shared_force_encoding
     s = "\u{3066}\u{3059}\u{3068}".gsub(//, '')
     h = {}