Don't return empty volname if volume is on unwanted vols list

When the volume was on the unwanted_list, the loop was
continued but the number of sql rows was not checked, so that
the code continued and returned an empty volumename.

Fixes #691: Backup job fails with "No Volume name given."

src/cats/sql_find.c

@@ -435,9 +435,18 @@ int db_find_next_volume(JCR *jcr, B_DB *mdb, int item, bool InChanger, MEDIA_DBR
       }
 
       /*
-       * See if this is not on the unwanted volumes list.
+       * Skip if volume is on unwanted volumes list.
+       * We need to reduce the number of rows by one
+       * and jump out if no more rows are available
        */
       if (unwanted_volumes && is_on_unwanted_volumes_list(row[1], unwanted_volumes)) {
+         Dmsg1(50, "Volume %s is on unwanted_volume_list, skipping\n", row[1]);
+         num_rows-- ;
+         if ( num_rows <= 0) {
+            Dmsg1(50, "No more volumes in result, bailing out\n", row[1]);
+            sql_free_result(mdb);
+            goto bail_out;
+         }
          continue;
       }