nwfilter: Fix double free of pointer

https://bugzilla.redhat.com/show_bug.cgi?id=1071181

Commit 49b59a15 fixed one problem but masks another one related to pointer
freeing.

Avoid putting of the virNWFilterSnoopReq once the thread has been started.
It belongs to the thread and the thread will call virNWFilterSnoopReqPut() on it.
Signed-off-by: NStefan Berger <stefanb@linux.vnet.ibm.com>

src/nwfilter/nwfilter_dhcpsnoop.c

@@ -1605,6 +1605,7 @@ virNWFilterDHCPSnoopReq(virNWFilterTechDriverPtr techdriver,
     int tmp;
     virThread thread;
     virNWFilterVarValuePtr dhcpsrvrs;
+    bool threadPuts = false;
 
     virNWFilterSnoopIFKeyFMT(ifkey, vmuuid, macaddr);
 
@@ -1698,6 +1699,8 @@ virNWFilterDHCPSnoopReq(virNWFilterTechDriverPtr techdriver,
         goto exit_snoopreq_unlock;
     }
 
+    threadPuts = true;
+
     virAtomicIntInc(&virNWFilterSnoopState.nThreads);
 
     req->threadkey = virNWFilterSnoopActivate(req);
@@ -1737,7 +1740,8 @@ exit_rem_ifnametokey:
 exit_snoopunlock:
     virNWFilterSnoopUnlock();
 exit_snoopreqput:
-    virNWFilterSnoopReqPut(req);
+    if (!threadPuts)
+        virNWFilterSnoopReqPut(req);
 
     return -1;
 }