BUG/MINOR: log: segfault if there are no proxy reference

The HAProxy API allow to send log without defined proxy (it
set to the NULL value). An incomplete test if done to choose
the log tag and an invalid pointer is dereferenced.
diff --git a/src/log.c b/src/log.c
index 6c01248..f7d160d 100644
--- a/src/log.c
+++ b/src/log.c
@@ -775,7 +775,7 @@
 	if (level < 0 || format == NULL || logline == NULL)
 		return;
 
-	dataptr = update_log_hdr(p->log_tag ? p->log_tag : global.log_tag); /* update log header and skip it */
+	dataptr = update_log_hdr(p && p->log_tag ? p->log_tag : global.log_tag); /* update log header and skip it */
 	data_len = dataptr - logline;
 
 	va_start(argp, format);