Cleanup EVP_DH asn1 parsing

[justsmth]

Description of changes:

Fixes several issues in crypto/evp_extra/p_dh_asn1.c:

1. dh_pub_decode: Fixed an unreachable double-free. After ownership of pubkey is transferred to the DH object via dh->pub_key = pubkey, the error path would free it twice — once through DH_free(dh) and again via BN_free(pubkey). This is currently unreachable because EVP_PKEY_assign_DH always succeeds when dh is non-NULL, but a future refactor could easily trigger it. Fixed by nulling the local pointer after the ownership transfer.

2. dh_pub_decode: Added a CBS_len(key) != 0 check after parsing the public key to reject trailing data, consistent with the RSA, DSA, and EC pub_decode implementations.

3. dh_param_copy: Fixed a NULL dereference when to->pkey.dh is NULL by allocating a new DH object on demand.

4. dh_pub_encode: Added an early NULL check for the DH object and its public key.

Call-outs:

These are defensive fixes hardening error paths and edge cases.

Testing:

Existing tests continue to pass.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and the ISC license.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 50.00000% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.35%. Comparing base (04e7dc0) to head (c3a42d6).

Files with missing lines	Patch %	Lines
crypto/evp_extra/p_dh_asn1.c	50.00%	5 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3047   +/-   ##
=======================================
  Coverage   78.35%   78.35%           
=======================================
  Files         689      689           
  Lines      121041   121050    +9     
  Branches    16968    16973    +5     
=======================================
+ Hits        94839    94854   +15     
+ Misses      25306    25300    -6     
  Partials      896      896           

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