Fix failing der test by gating vec! use behind alloc #1322

[kpcyrd]

This seems to address the failing test described in #1322.

I've tested this using:

cargo test -F der
cargo test -F der -F alloc

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.04%. Comparing base (e25b054) to head (8c39146).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1323   +/-   ##
=======================================
  Coverage   91.04%   91.04%           
=======================================
  Files         189      189           
  Lines       22598    22598           
=======================================
  Hits        20574    20574           
  Misses       2024     2024           

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