Fixes segfault in beam quanting. (issue4339047)

[mtsolo]

Reviewers: ,

Message:
Quick-n-dirty fix for segfault in issue 1585.

Cheers,
MS

Description:
Fixes segfault in beam quanting.

Please review this at http://codereview.appspot.com/4339047/

Affected files:
  M lily/beam-quanting.cc


Index: lily/beam-quanting.cc
diff --git a/lily/beam-quanting.cc b/lily/beam-quanting.cc
index  
e868e51d7f124af9c4ee4a690a19890314569429..c33afdb34ed83df40175f8051fb1f2eda9c3c984  
100644
--- a/lily/beam-quanting.cc
+++ b/lily/beam-quanting.cc
@@ -459,6 +459,9 @@ Beam_scoring_problem::solve () const {
     }
   else
     {
+      if (!configs.size())
+        return unquanted_y;
+
       std::priority_queue<Beam_configuration*,  
std::vector<Beam_configuration*>,
                           Beam_configuration_less> queue;
       for (vsize i = 0; i < configs.size(); i++)