conf: fix memory leak of class_id bitmap

When libvirtd loads active network configs from network state directory,
it should release the class_id memory block which was allocated
at the time of loading xml from network config directory.
virBitmapParse will create a new memory block of bitmap class_id which
causes a memory leak.

This happens when at least one virtual network is active before.

==12234== 8,216 (24 direct, 8,192 indirect) bytes in 1 blocks are definitely \
              lost in loss record 702 of 709
==12234==    at 0x4A06B2F: calloc (vg_replace_malloc.c:593)
==12234==    by 0x37AB04D77D: virAlloc (in /usr/lib64/libvirt.so.0.1000.3)
==12234==    by 0x37AB04EF89: virBitmapNew (in /usr/lib64/libvirt.so.0.1000.3)
==12234==    by 0x37AB0BFB37: virNetworkAssignDef (in /usr/lib64/libvirt.so.0.1000.3)
==12234==    by 0x37AB0BFD31: ??? (in /usr/lib64/libvirt.so.0.1000.3)
==12234==    by 0x37AB0BFE92: virNetworkLoadAllConfigs (in /usr/lib64/libvirt.so.0.1000.3)
==12234==    by 0x10650E5A: ??? (in /usr/lib64/libvirt/connection-driver/libvirt_driver_network.so)
==12234==    by 0x37AB0EB72F: virStateInitialize (in /usr/lib64/libvirt.so.0.1000.3)
==12234==    by 0x40DE04: ??? (in /usr/sbin/libvirtd)
==12234==    by 0x37AB0832E8: ??? (in /usr/lib64/libvirt.so.0.1000.3)
==12234==    by 0x3796807D14: start_thread (in /usr/lib64/libpthread-2.16.so)
==12234==    by 0x37960F246C: clone (in /usr/lib64/libc-2.16.so)

src/conf/network_conf.c

@@ -1995,14 +1995,16 @@ virNetworkObjUpdateParseFile(const char *filename,
 
         ctxt->node = node;
         class_id = virXPathString("string(./class_id[1]/@bitmap)", ctxt);
-        if (class_id &&
-            virBitmapParse(class_id, 0,
-                           &net->class_id, CLASS_ID_BITMAP_SIZE) < 0) {
-            virReportError(VIR_ERR_INTERNAL_ERROR,
-                           _("Malformed 'class_id' attribute: %s"),
-                           class_id);
-            VIR_FREE(class_id);
-            goto cleanup;
+        if (class_id) {
+            virBitmapFree(net->class_id);
+            if (virBitmapParse(class_id, 0,
+                               &net->class_id, CLASS_ID_BITMAP_SIZE) < 0) {
+                virReportError(VIR_ERR_INTERNAL_ERROR,
+                               _("Malformed 'class_id' attribute: %s"),
+                               class_id);
+                VIR_FREE(class_id);
+                goto cleanup;
+            }
         }
         VIR_FREE(class_id);