input-raw: Fix wrong first slot values by using calloc instead of malloc

We don't allocate memory often, so this is no performance issue.
And this fixes the case when the driver
doesn't send ABS_MT_SLOT 0 on the first pen-down. We simply start
at slot 0 implicitely and actually should expect to start off with
zeroes in our buffer.

plugins/input-raw.c

@@ -560,7 +560,7 @@ static int ts_input_read_mt(struct tslib_module_info *inf,
 			return -ENOMEM;
 
 		for (j = 0; j < nr; j++) {
-			i->buf[j] = malloc(max_slots *
+			i->buf[j] = calloc(max_slots,
 					   sizeof(struct ts_sample_mt));
 			if (!i->buf[j])
 				return -ENOMEM;