namecache: fix NULL-pointer dereference in namecache-flat

namecache-flat crashes if started on an empty or non-existent file.
gnunet-service-[2568]: segfault at 0 ip 00007fe32dc8bb3c sp 00007ffde11826f0 error 4 in libgnunet_plugin_namecache_flat.so[7fe32dc8a000+202000]
gnunet-service-[2576]: segfault at 0 ip 00007f2a6be75b3c sp 00007ffdf9b6d880 error 4 in libgnunet_plugin_namecache_flat.so[7f2a6be74000+202000]
gnunet-service-[2581]: segfault at 0 ip 00007ff1dcfb1b3c sp 00007fff834070f0 error 4 in libgnunet_plugin_namecache_flat.so[7ff1dcfb0000+202000]

This fix at least prevents parsing a 0-byte file, however, looking at
the code it is clear that more needs to be done to prevent crashes in
case of corrupted or incompletely written cache data.

Fixes https://gnunet.org/bugs/view.php?id=4082

Signed-off-by: Daniel Golle <daniel@makrotopia.org>

1 files changed, 3 insertions, 0 deletions

diff --git a/src/namecache/plugin_namecache_flat.c b/src/namecache/plugin_namecache_flat.c
index 409b05802..85bb69237 100644
--- a/src/namecache/plugin_namecache_flat.c
+++ b/src/namecache/plugin_namecache_flat.c
@@ -132,6 +132,9 @@ database_setup (struct Plugin *plugin)
     return GNUNET_SYSERR;
   }
 
+  if (0 == size)
+    return GNUNET_OK;
+
   buffer = GNUNET_malloc (size);
 
   if (GNUNET_SYSERR == GNUNET_DISK_file_read (fh,