DVB-S2-BB: Prevent infinite loop

Commit 4bf4ee88f0 removed an else
statement that broke out of the BBFrame processing loop. Without
it, infinite loops might be possible if the GSE frames have bit errors
in the length field.

epan/dissectors/packet-dvb-s2-bb.c

@@ -1257,6 +1257,8 @@ static int dissect_dvb_s2_bb(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree
                         bb_data_len -= sub_dissected;
                         if (bb_data_len < DVB_S2_GSE_MINSIZE)
                             bb_data_len = 0;
+                    } else {
+                        bb_data_len = 0;
                     }
                 }
             }