A fix for a Fedora 28 bug

I just found this Pan bug after upgrading to Fedora 28. It's because of
new compilers and C++ libraries, etc. And because calling front() on an
empty container is a bug.

Patch:

From 564d931d4f88eaea18bb30abd7ffd8b4ba468bc8 Mon Sep 17 00:00:00 2001
From: Jonathan Briggs <[email protected]>
Date: Tue, 3 Apr 2018 23:29:20 -0600
Subject: [PATCH] Do not use front() on empty vector.

---
 pan/data-impl/rules-filter.cc | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/pan/data-impl/rules-filter.cc b/pan/data-impl/rules-filter.cc
index abc2506..c0d26c4 100644
--- a/pan/data-impl/rules-filter.cc
+++ b/pan/data-impl/rules-filter.cc
@@ -36,7 +36,9 @@ RulesFilter :: finalize (Data& data)
   _delete.clear();

   const std::vector<const Article*> tmp (_mark_read.begin(),
_mark_read.end());
-  data.mark_read ((const Article**)&tmp.front(), tmp.size());
+  if (!tmp.empty()) {
+    data.mark_read ((const Article**)&tmp.front(), tmp.size());
+  }
   _mark_read.clear();

   const std::vector<const Article*> tmp2 ( _cached.begin(),
_cached.end());
-- 
2.17.0

-- 
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                        Study Hard -- Be Evil