[pan2] Bug 793228 nzb file with empty segment lists crash pan

[Petr Kovář <pmkovar src gnome org>]

commit 29aad7ec8b2b0c74592342fcfa2aa3c830f9e592
Author: paul flinders org <paul flinders org>
Date:   Thu Feb 8 21:28:07 2018 +0100

    Bug 793228 nzb file with empty segment lists crash pan
    
    NZB files which have empty segment lists crash pan (up to 0.144),
    for me typically at the assertion on the last line of
    Parts :: set_parts (around parts.cc line 237).
    
    The problem is that "mc.parts" is not re-initialised for the new
    list of segments if the first segment is not seen, depending on
    the position in the file you will either get an incompletely
    constructed PartBatch used (PartBatch relies on a default
    constructor so packed_mids_len doesn't get initialised) or an
    old list of segments could get re-used.

 pan/tasks/nzb.cc |    6 +++++-
 1 files changed, 5 insertions(+), 1 deletions(-)
---
diff --git a/pan/tasks/nzb.cc b/pan/tasks/nzb.cc
index e3e8547..af890e5 100644
--- a/pan/tasks/nzb.cc
+++ b/pan/tasks/nzb.cc
@@ -113,6 +113,8 @@ namespace
                       gpointer             user_data,
                       GError             **error           UNUSED)
   {
+    static Quark null_mid;
+
     MyContext& mc (*static_cast<MyContext*>(user_data));
 
     if (!strcmp (element_name, "file")) {
@@ -123,7 +125,9 @@ namespace
         else if (!strcmp (*k,"date"))    mc.a.time_posted = strtoul(*v,0,10);
       }
     }
-
+    else if (!strcmp (element_name, "segments")) {
+        mc.parts.init (null_mid);
+    }
     else if (!strcmp (element_name, "segment")) {
       mc.bytes = 0;
       mc.number = 0;