BUG/MINOR: cfgparse-tcp: leak when re-declaring interface from bind line

BUG/MINOR: cfgparse-tcp: leak when re-declaring interface from bind line

When interface keyword is used multiple times within the same bind line,
the previous value isn't checked and is rewritten as-is, resulting in a
small memory leak.

Ensuring the interface name is first freed before assigning it to a new
value.

This may be backported to every stable versions.

[Note for 2.2, the fix must be performed in bind_parse_interface() from
proto_tcp.c, directly within the listener's loop, also ha_free() was
not available so free() must be used instead]