osmux: initialize osmux_batch_size in mgcp config

The library allows to indicate zero as batch size if you want to use
the default size, however openbsc saves 'osmux batch-size 0' which is
not good as input.

Use OSMUX_BATCH_DEFAULT_MAX to explicitly initialize the batch size
from mgcp_parse_config().

openbsc/src/libmgcp/mgcp_vty.c

@@ -1307,8 +1307,8 @@ int mgcp_parse_config(const char *config_file, struct mgcp_config *cfg,
 	struct mgcp_trunk_config *trunk;
 
 	cfg->osmux_port = OSMUX_PORT;
-	/* Default to 4 messages */
 	cfg->osmux_batch = 4;
+	cfg->osmux_batch_size = OSMUX_BATCH_DEFAULT_MAX;
 
 	g_cfg = cfg;
 	rc = vty_read_config_file(config_file, NULL);