BUG/MINOR: cfgparse: Fail if the strdup() for `rule->be.name` for `use_backend` fails

BUG/MINOR: cfgparse: Fail if the strdup() for `rule->be.name` for `use_backend` fails

This patch fixes GitHub issue #1024.

I could track the `strdup` back to commit
3a1f5fda109fc56ae127d03eaf34ce027c9542e1 which is 1.9-dev8. It's probably not
worth the effort to backport it across this refactoring.

This patch should be backported to 1.9+.

(cherry picked from commit 5ce5a1586d8c67affa604dedf0faa2c5a02ecab5)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit f46d9cd735d1664c113c041d85179a81bded8fc6)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit fdca7df8181de29a661be4e8f91160b087843167)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>