ext/socket/init.c: do not return uninitialized buffer

Resize string buffer only if some data is received in
BasicSocket#read_nonblock and some methods.

Co-Authored-By: Samuel Williams <samuel.williams@oriontransfer.co.nz>



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_6@67875 b2dd03c8-39d4-4d8f-98ff-823fe69b080e