Fixed heap-use-after-free

* string.c (rb_str_sub_bang): retrieves a pointer to the
  replacement string buffer just before using it, for the case of
  replacement with the receiver string itself.  [Bug #16105]

string.c

@@ -5098,7 +5098,7 @@ rb_str_sub_bang(int argc, VALUE *argv, VALUE str)
                 cr = cr2;
 	}
 	plen = end0 - beg0;
-	rp = RSTRING_PTR(repl); rlen = RSTRING_LEN(repl);
+        rlen = RSTRING_LEN(repl);
 	len = RSTRING_LEN(str);
 	if (rlen > plen) {
 	    RESIZE_CAPA(str, len + rlen - plen);
@@ -5107,6 +5107,7 @@ rb_str_sub_bang(int argc, VALUE *argv, VALUE str)
 	if (rlen != plen) {
 	    memmove(p + beg0 + rlen, p + beg0 + plen, len - beg0 - plen);
 	}
+        rp = RSTRING_PTR(repl);
         memmove(p + beg0, rp, rlen);
 	len += rlen - plen;
 	STR_SET_LEN(str, len);

test/ruby/test_string.rb

@@ -2010,6 +2010,12 @@ CODE
     r.taint
     a.sub!(/./, r)
     assert_predicate(a, :tainted?)
+
+    bug16105 = '[Bug #16105] heap-use-after-free'
+    a = S("ABCDEFGHIJKLMNOPQRSTUVWXYZ012345678")
+    b = a.dup
+    c = a.slice(1, 100)
+    assert_equal("AABCDEFGHIJKLMNOPQRSTUVWXYZ012345678", b.sub!(c, b), bug16105)
   end
 
   def test_succ