crypto/evp: fix double free of tmp_keymgmt in sig/kem/asym init

Commit ecb4757b377f "crypto/evp/m_sigver.c: fix potential double free
on error path in do_sigver_init" has fixed double-free of tmp_keymgmt
in do_sigver_init() by setting it to NULL after EVP_KEYMGMT_free() call;
the same issue present in evp_kem_init(), evp_pkey_asym_cipher_init(),
and evp_pkey_signature_init().  Address it similarly, by setting
the pointers to NULL after *_free() calls.

Complements: ecb4757b377f "crypto/evp/m_sigver.c: fix potential double free on error path in do_sigver_init"
Fixes: 839ffdd11cd4 "EVP: Allow a fallback for operations that work with an EVP_PKEY"
CLA: trivial

Reviewed-by: Kurt Roeckx <kurt@roeckx.be>
Reviewed-by: Tomas Mraz <tomas@openssl.foundation>
Reviewed-by: Eugene Syromiatnikov <esyr@openssl.org>
MergeDate: Sun May 31 11:03:15 2026
(Merged from https://github.com/openssl/openssl/pull/31312)