BUG/MINOR: extcheck: add missing checks on extchk_setenv()

Issue #910 reports that we fail to check a few extchk_setenv() in the
child process. These are mostly harmless, but instead of counting on
the external check script to fail the dirty way, better fail cleanly
when detecting the failure.

This could probably be backported to all stable branches.
diff --git a/src/extcheck.c b/src/extcheck.c
index 96d61c9..6665e96 100644
--- a/src/extcheck.c
+++ b/src/extcheck.c
@@ -427,20 +427,21 @@
 		environ = check->envp;
 
 		/* Update some environment variables and command args: curconn, server addr and server port */
-		extchk_setenv(check, EXTCHK_HAPROXY_SERVER_CURCONN, ultoa_r(s->cur_sess, buf, sizeof(buf)));
+		EXTCHK_SETENV(check, EXTCHK_HAPROXY_SERVER_CURCONN, ultoa_r(s->cur_sess, buf, sizeof(buf)), fail);
 
 		addr_to_str(&s->addr, check->argv[3], EXTCHK_SIZE_ADDR);
-		extchk_setenv(check, EXTCHK_HAPROXY_SERVER_ADDR, check->argv[3]);
+		EXTCHK_SETENV(check, EXTCHK_HAPROXY_SERVER_ADDR, check->argv[3], fail);
 
 		*check->argv[4] = 0;
 		if (s->addr.ss_family == AF_INET || s->addr.ss_family == AF_INET6)
 			snprintf(check->argv[4], EXTCHK_SIZE_UINT, "%u", s->svc_port);
-		extchk_setenv(check, EXTCHK_HAPROXY_SERVER_PORT, check->argv[4]);
+		EXTCHK_SETENV(check, EXTCHK_HAPROXY_SERVER_PORT, check->argv[4], fail);
 
 		haproxy_unblock_signals();
 		execvp(px->check_command, check->argv);
 		ha_alert("Failed to exec process for external health check: %s. Aborting.\n",
 			 strerror(errno));
+	fail:
 		exit(-1);
 	}