Always initialize the output parameters

I/O functions with output parameters, should always initialize those
output parameters, even when an error is returned. This prevents the
(accidental) use of uninitialized variables, whenever the caller forgets
to check the return code.

As a nice side effect, the use of a local variable guarantees that the
underlying I/O implementation will always receive a valid pointer.

src/iostream.c

@@ -131,19 +131,41 @@ dc_iostream_set_rts (dc_iostream_t *iostream, unsigned int value)
 dc_status_t
 dc_iostream_get_lines (dc_iostream_t *iostream, unsigned int *value)
 {
-	if (iostream == NULL || iostream->vtable->get_lines == NULL)
+	dc_status_t status = DC_STATUS_SUCCESS;
-		return DC_STATUS_UNSUPPORTED;
+	unsigned int lines = 0;
 
-	return iostream->vtable->get_lines (iostream, value);
+	if (iostream == NULL || iostream->vtable->get_lines == NULL) {
+		status = DC_STATUS_UNSUPPORTED;
+		goto out;
+	}
+
+	status = iostream->vtable->get_lines (iostream, &lines);
+
+out:
+	if (value)
+		*value = lines;
+
+	return status;
 }
 
 dc_status_t
 dc_iostream_get_available (dc_iostream_t *iostream, size_t *value)
 {
-	if (iostream == NULL || iostream->vtable->get_available == NULL)
+	dc_status_t status = DC_STATUS_SUCCESS;
-		return DC_STATUS_UNSUPPORTED;
+	size_t available = 0;
 
-	return iostream->vtable->get_available (iostream, value);
+	if (iostream == NULL || iostream->vtable->get_available == NULL) {
+		status = DC_STATUS_UNSUPPORTED;
+		goto out;
+	}
+
+	status = iostream->vtable->get_available (iostream, &available);
+
+out:
+	if (value)
+		*value = available;
+
+	return status;
 }
 
 dc_status_t