Use ALLOCV to allocate struct crypt_data for slightly cleaner and less
error-prone code. It is currently possible it leaks when an invalid
argument is passed to String#crypt or rb_str_new_cstr() fails to
allocate memory.

SIZEOF_CRYPT_DATA macro in missing/crypt.h is removed since it is not
used any longer.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@60748 b2dd03c8-39d4-4d8f-98ff-823fe69b080e