MT#56128 SDP manipulations: CMD_REM check emptiness of attribute

Do not proceed to the `g_hash_table_lookup()` if the `attr_name`
is empty.

Additonally: improve slightly the same check for the CMD_SUBST.

Change-Id: I39e34c34b458150ebae38dff5e6a46f6e8f17c66

daemon/sdp.c

@@ -324,7 +324,7 @@ static int sdp_manipulate_check(enum command_type command_type,
 		case CMD_SUBST:;
 			q_ptr = NULL;
 
-			if (!attr_name)
+			if (!attr_name || !attr_name->len)
 				break;
 
 			struct sdp_substitute_attr fictitious = {attr_name, NULL};
@@ -361,6 +361,10 @@ static int sdp_manipulate_check(enum command_type command_type,
 
 		case CMD_REM:;
 			GHashTable *ht = NULL;
+
+			if (!attr_name || !attr_name->len)
+				break;
+
 			switch (media_type) {
 				case MT_AUDIO:
 					ht = sdp_manipulations->rem_commands_audio;