Fixes the assert problem coming from ledger-line-spanner.cc. (issue4535055)

[mtsolo]

Reviewers: ,

Message:
This seems like a quick, easy, and reasonable fix for Issue 1652.  Lemme
know if it's OK!

Cheers,
MS

Description:
Fixes the assert problem coming from ledger-line-spanner.cc.

Please review this at http://codereview.appspot.com/4535055/

Affected files:
  M lily/ledger-line-spanner.cc


Index: lily/ledger-line-spanner.cc
diff --git a/lily/ledger-line-spanner.cc b/lily/ledger-line-spanner.cc
index  
49ff7119116f85e45a027f3fab630edc7905b702..a518bc9225b5ef0abdd819cb42436c8336d6cda5  
100644
--- a/lily/ledger-line-spanner.cc
+++ b/lily/ledger-line-spanner.cc
@@ -320,7 +320,7 @@ Ledger_line_spanner::print (SCM smob)
       Item *h = dynamic_cast<Item *> (heads[i]);

       int pos = Staff_symbol_referencer::get_rounded_position (h);
-      if (!staff_extent.contains (pos - sign (pos)))
+      if (!staff_extent.contains (pos - sign (pos))  
&& !staff_extent.is_empty ())
        {
          Interval head_size = h->extent (common[X_AXIS], X_AXIS);
          Interval ledger_size = head_size;