merge revision(s) 43853: [Backport #9157]

* file.c (rb_readlink): fix buffer overflow on a long symlink. since
	  rb_str_modify_expand() expands from its length but not its capacity,
	  need to set the length properly for each expansion.
	  [ruby-core:58592] [Bug #9157]


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_0_0@43959 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

file.c

@@ -2531,6 +2531,7 @@ rb_readlink(VALUE path)
 	) {
 	rb_str_modify_expand(v, size);
 	size *= 2;
+	rb_str_set_len(v, size);
     }
     if (rv < 0) {
 	rb_str_resize(v, 0);