logger.c: Fix buffer overrun found by address sanitizer.

The null terminator of the tail struct member was not being allocated
when no logger.conf config file is installed.

ASTERISK-25714 #close
Reported by: Badalian Vyacheslav

Change-Id: I45770fdd08af39506a3bc33ba279c4f16e047a30

main/logger.c

@@ -601,7 +601,7 @@ static int init_logger_chain(const char *altconf)
 
 	/* If no config file, we're fine, set default options. */
 	if (!cfg) {
-		if (!(chan = ast_calloc(1, sizeof(*chan)))) {
+		if (!(chan = ast_calloc(1, sizeof(*chan) + 1))) {
 			fprintf(stderr, "Failed to initialize default logging\n");
 			return -1;
 		}