message: fix crash in bv2float with invalid expr
diff: @@ -1107,7 +1107,7 @@ expr expr::float2Real() const {
  }
  expr expr::BV2float(const expr &type) const {
  - C2(type);
  + C(type);
  if (auto app = isAppOf(Z3_OP_FPA_TO_IEEE_BV)) {
  expr arg = Z3_get_app_arg(ctx(), app, 0);
  if (arg.sort() == type.sort())
  