tcp_connection.pony: fix _read_buf_size() realloc bug
tcp_connection.pony: fix _read_buf_size() realloc bug

Fixes #3182

When _expect != 0, then we know how many bytes to expect from
the socket, so only call _read_buf.undefined() when we know
that the buffer is too small to accomodate _expect bytes.

kulibali/ponyc,sgebbie/ponyc,sgebbie/ponyc,cquinn/ponyc,ponylang/ponyc,dipinhora/ponyc,sgebbie/ponyc,jemc/ponyc,dipinhora/ponyc,jemc/ponyc,cquinn/ponyc,sgebbie/ponyc,kulibali/ponyc,sgebbie/ponyc,jemc/ponyc,cquinn/ponyc,ponylang/ponyc,kulibali/ponyc,kulibali/ponyc,cquinn/ponyc,ponylang/ponyc,dipinhora/ponyc