pfrd: fix mod by zero error when all branches are filtered

src/policy_eppfrd.cpp

@@ -155,6 +155,9 @@ namespace eppfrd
     uint64_t idx;
     uint64_t threshold;
 
+    if(sum_ == 0)
+      return NULL;
+
     idx = 0;
     threshold = RND::rand64(sum_);
     for(size_t i = 0; i < branchinfo_.size(); i++)

src/policy_msppfrd.cpp

@@ -114,6 +114,9 @@ namespace msppfrd
     uint64_t idx;
     uint64_t threshold;
 
+    if(sum_ == 0)
+      return NULL;
+
     idx = 0;
     threshold = RND::rand64(sum_);
     for(size_t i = 0; i < branchinfo_.size(); i++)

src/policy_pfrd.cpp

@@ -84,6 +84,9 @@ namespace pfrd
     uint64_t idx;
     uint64_t threshold;
 
+    if(sum_ == 0)
+      return NULL;
+
     idx = 0;
     threshold = RND::rand64(sum_);
     for(size_t i = 0; i < branchinfo_.size(); i++)