Avoid returning enum in boolean context

[dkg]

Without this fix, when compiling with ENABLE_CRYPTO_REFRESH, we see:

…/src/lib/enc_material.cpp: In member function ‘virtual bool pgp::X25519EncMaterial::parse(pgp_packet_body_t&)’:
…/src/lib/enc_material.cpp:154:20: warning: enum constant in boolean context [-Wint-in-bool-context]
  154 |             return RNP_ERROR_BAD_FORMAT;
      |                    ^~~~~~~~~~~~~~~~~~~~
make[3]: Leaving directory '…/build'

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 85.46%. Comparing base (bf89f0b) to head (dcaa3a1).

Additional details and impacted files

@@             Coverage Diff              @@
##           release/0.x    #2364   +/-   ##
============================================
  Coverage        85.46%   85.46%           
============================================
  Files              126      126           
  Lines            22710    22710           
============================================
  Hits             19408    19408           
  Misses            3302     3302           

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[TJ-91]

FYI, this, among other things, is also being fixed in PR https://github.com/rnpgp/rnp/pull/2355/