libnm-core: Add a missing initializer

/libnm/crypto/PKCS#8: OK
/libnm/crypto/cert/pem: ==16241== Conditional jump or move depends on uninitialised value(s)
==16241==    at 0x121967: crypto_load_and_verify_certificate (crypto.c:605)
==16241==    by 0x12512E: nm_utils_file_is_certificate (nm-utils.c:2279)
==16241==    by 0x1201F1: test_cert (test-crypto.c:113)