Fix regression on vat calculation when invoice was not a situation

invoice

htdocs/accountancy/journal/sellsjournal.php

@@ -142,12 +142,13 @@ if ($result) {
 		// Situation invoices handling
 		$line = new FactureLigne($db);
 		$line->fetch($obj->fdid); // id of line
-		$prev_progress = $line->get_prev_progress($obj->rowid);   // id of invoice
+		$prev_progress = 0;
 		if ($obj->type == Facture::TYPE_SITUATION) {
 		    // Avoid divide by 0
 			if ($obj->situation_percent == 0) {
 				$situation_ratio = 0;
 			} else {
+		        $prev_progress = $line->get_prev_progress($obj->rowid);   // id of invoice
 			    $situation_ratio = ($obj->situation_percent - $prev_progress) / $obj->situation_percent;
 			}
 		} else {

htdocs/compta/journal/sellsjournal.php

@@ -167,12 +167,13 @@ if ($result)
 		// Situation invoices handling
 		$line = new FactureLigne($db);
 		$line->fetch($obj->id);   // id of line
-		$prev_progress = $line->get_prev_progress($obj->rowid);   // id on invoice
+		$prev_progress = 0;
 		if ($obj->type==Facture::TYPE_SITUATION) {
 			// Avoid divide by 0
 			if ($obj->situation_percent == 0) { 
 				$situation_ratio = 0;
 			} else {
+		        $prev_progress = $line->get_prev_progress($obj->rowid);   // id on invoice
 			    $situation_ratio = ($obj->situation_percent - $prev_progress) / $obj->situation_percent;
 			}
 		} else {