CLEANUP: admin/halog: improve handling of memory allocation errors

Found via cppcheck  --force --enable=all --output-file=haproxy.log :

admin/halog/halog.c:1805:2: warning: If memory allocation fails, then there is a possible null pointer dereference: ustat [nullPointerOutOfMemory]
admin/halog/halog.c:1806:2: warning: If memory allocation fails, then there is a possible null pointer dereference: ustat [nullPointerOutOfMemory]
admin/halog/halog.c:1809:2: warning: If memory allocation fails, then there is a possible null pointer dereference: ustat [nullPointerOutOfMemory]
admin/halog/halog.c:1810:2: warning: If memory allocation fails, then there is a possible null pointer dereference: ustat [nullPointerOutOfMemory]
admin/halog/halog.c:1814:2: warning: If memory allocation fails, then there is a possible null pointer dereference: ustat [nullPointerOutOfMemory]

Author: chipitsine

admin/halog/halog.c

@@ -1801,6 +1801,8 @@ void filter_count_ip(const char *source_field, const char *accept_field, const c
 	 */
 	if (unlikely(!ustat))
 		ustat = calloc(1, sizeof(*ustat));
+	if (!ustat)
+		return;
 
 	ustat->nb_err = err;
 	ustat->nb_req = 1;