ofproto: Fix potential NULL dereference in ofproto_get_datapath_cap().

Reproducer:
  ovs-vsctl \
    -- add-br br \
    -- set bridge br datapath-type=foo \
    -- --id=@m create Datapath datapath_version=0 'capabilities={}' \
    -- set Open_vSwitch . datapaths:"foo"=@m

Fixes: 2750180 ("ofproto-dpif: Expose datapath capability to ovsdb.")
Signed-off-by: Dumitru Ceara <[email protected]>
Acked-by: Paolo Valerio <[email protected]>
Signed-off-by: Ilya Maximets <[email protected]>

ofproto/ofproto.c

@@ -968,7 +968,7 @@ ofproto_get_datapath_cap(const char *datapath_type, struct smap *dp_cap)
     datapath_type = ofproto_normalize_type(datapath_type);
     const struct ofproto_class *class = ofproto_class_find__(datapath_type);
 
-    if (class->get_datapath_cap) {
+    if (class && class->get_datapath_cap) {
         class->get_datapath_cap(datapath_type, dp_cap);
     }
 }