plugins: filters: check for read_mt() availability

In case users use raw modules that don't implement ts_read(), we must fail
gracefully.

plugins/debounce.c

@@ -131,6 +131,9 @@ static int debounce_read_mt(struct tslib_module_info *info, struct ts_sample_mt
 		p->current_max_slots = max_slots;
 	}
 
+	if (!info->next->ops->read_mt)
+		return -ENOSYS;
+
 	ret = info->next->ops->read_mt(info->next, samp, max_slots, nr_samples);
 	if (ret < 0)
 		return ret;

plugins/dejitter.c

@@ -193,6 +193,9 @@ static int dejitter_read_mt(struct tslib_module_info *info,
 	int ret;
 	int i, j;
 
+	if (!info->next->ops->read_mt)
+		return -ENOSYS;
+
 	ret = info->next->ops->read_mt(info->next, samp, max_slots, nr);
 	if (ret < 0)
 		return ret;

plugins/iir.c

@@ -132,6 +132,9 @@ static int iir_read_mt(struct tslib_module_info *info,
 		iir->slots = max_slots;
 	}
 
+	if (!info->next->ops->read_mt)
+		return -ENOSYS;
+
 	ret = info->next->ops->read_mt(info->next, samp, max_slots, nr);
 	if (ret < 0)
 		return ret;

plugins/linear.c

@@ -94,6 +94,9 @@ static int linear_read_mt(struct tslib_module_info *info,
 	int i;
 	int nr;
 
+	if (!info->next->ops->read_mt)
+		return -ENOSYS;
+
 	ret = info->next->ops->read_mt(info->next, samp, max_slots, nr_samples);
 	if (ret < 0)
 		return ret;

plugins/median.c

@@ -199,6 +199,9 @@ static int median_read_mt(struct tslib_module_info *inf,
 	int ret;
 	int i, j;
 
+	if (!inf->next->ops->read_mt)
+		return -ENOSYS;
+
 	ret = inf->next->ops->read_mt(inf->next, samp, max_slots, nr);
 	if (ret < 0)
 		return ret;

plugins/pthres.c

@@ -143,6 +143,9 @@ static int pthres_read_mt(struct tslib_module_info *info,
 		p->current_max_slots = max_slots;
 	}
 
+	if (!info->next->ops->read_mt)
+		return -ENOSYS;
+
 	ret = info->next->ops->read_mt(info->next, samp, max_slots, nr_samples);
 	if (ret < 0)
 		return ret;

plugins/skip.c

@@ -236,6 +236,9 @@ static int skip_read_mt(struct tslib_module_info *info,
 		skip->slots = max_slots;
 	}
 
+	if (!info->next->ops->read_mt)
+		return -ENOSYS;
+
 	ret = info->next->ops->read_mt(info->next, samp, max_slots, nr);
 	if (ret < 0)
 		return ret;

plugins/variance.c

@@ -150,6 +150,9 @@ static int variance_read_mt(struct tslib_module_info *info,
 	short pen_down = 1;
 	int ret;
 
+	if (!info->next->ops->read_mt)
+		return -ENOSYS;
+
 	if (var->samp == NULL) {
 		var->samp = calloc(nr, sizeof(struct ts_sample));
 		if (!var->samp)