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>
diff --git a/src/cfgparse-listen.c b/src/cfgparse-listen.c
index 97a97e7..18e6770 100644
--- a/src/cfgparse-listen.c
+++ b/src/cfgparse-listen.c
@@ -1509,6 +1509,10 @@
 		}
 		rule->cond = cond;
 		rule->be.name = strdup(args[1]);
+		if (!rule->be.name) {
+			ha_alert("Out of memory error.\n");
+			goto out;
+		}
 		rule->line = linenum;
 		rule->file = strdup(file);
 		if (!rule->file) {