BUG/MINOR: log: Detect missing sampling ranges in config

Consider a config like:

    global
    	log 127.0.0.1:10001 sample :10 local0

No sampling ranges are given here, leading to NULL being passed
as the first argument to qsort.

This configuration does not make sense anyway, a log without ranges
would never log. Thus output an error if no ranges are given.

This bug was introduced in d95ea2897e.
This fix must be backported to HAProxy 2.0.

src/log.c

@@ -929,6 +929,11 @@ int parse_logsrv(char **args, struct list *logsrvs, int do_del, char **err)
 			smp_rgs_sz++;
 		}
 
+		if (smp_rgs == NULL) {
+			memprintf(err, "no sampling ranges given");
+			goto error;
+		}
+
 		beg = smp_sz_str;
 		end = beg + strlen(beg);
 		new_smp_sz = read_uint((const char **)&beg, end);