musicexp.py: Fix for issue 1985 (issue 5303063)

[ptrcklschmdt]

Reviewers: Reinhold,

Message:
I uploaded a patch for Issue 1985: musicxml2ly: group-symbol none leads
to bus error.

Description:
musicexp.py: Fix for issue 1985
The musicxml value 'none' of the group-symbol element is now converted
to 'SystemStartBar' instead of 'f', the compilation of which lead to a
bus error/seg fault.

Please review this at http://codereview.appspot.com/5303063/

Affected files:
  M python/musicexp.py


Index: python/musicexp.py
diff --git a/python/musicexp.py b/python/musicexp.py
index  
d105ef333d2bfb8f50f2dba428b4fbb00caf29e0..8610f8aa44f138886c5a1bed5cb4963a3f43a34f  
100644
--- a/python/musicexp.py
+++ b/python/musicexp.py
@@ -1793,7 +1793,7 @@ class StaffGroup:
         if self.spanbar == "no":
             printer.dump ("\\override SpanBar #'transparent = ##t")
         brack = {"brace": "SystemStartBrace",
-                 "none": "f",
+                 "none": "SystemStartBar",
                  "line": "SystemStartSquare"}.get (self.symbol, None)
         if brack:
             printer.dump ("systemStartDelimiter = #'%s" % brack)