BUG/MINOR: config: fix leak on proxy.conn_src.bind_hdr_name

BUG/MINOR: config: fix leak on proxy.conn_src.bind_hdr_name

Leak for parsing of option usesrc of the source keyword.

This can be backported to 1.8.

(cherry picked from commit 69c5c3ab330584f9c53e2cf7c86af371a84f104d)
Signed-off-by: Willy Tarreau <w@1wt.eu>
(cherry picked from commit 7edf20eb55282d86e78fa1c642975b4b8ffcbec4)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit 50c1fa2e4c59e097b10ee4d7fab699e8a8a5e73b)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>