do not break if there are no nets loaded from sym section

ice40/bitstream.cc

@@ -870,12 +870,14 @@ bool read_asc(Context *ctx, std::istream &in)
             }
             if (isUsed) {
                 NetInfo *net = ctx->wire_to_net[pi.dst];
+                if (net!=nullptr) {
                     WireId wire;
                     wire.index = pi.dst;
                     ctx->unbindWire(wire);
                     ctx->bindPip(pip, net, STRENGTH_WEAK);
                 }
             }
+        }
         for (auto bel : ctx->getBels()) {
             if (ctx->getBelType(bel) == id_ICESTORM_LC) {
                 const TileInfoPOD &ti = bi.tiles_nonrouting[TILE_LOGIC];