BUG/MEDIUM: jwe: fix NULL deref crash with empty CEK and non-dir alg

BUG/MEDIUM: jwe: fix NULL deref crash with empty CEK and non-dir alg

In sample_conv_jwt_decrypt_secret(), when a JWE token has an empty
encrypted-key section but the algorithm is not "dir" (e.g. A128KW),
neither branch initializes decrypted_cek. The NULL pointer is then
passed to decrypt_ciphertext() which dereferences it:

  - For GCM encodings: aes_process() calls b_orig(NULL) -> SIGSEGV
  - For CBC encodings: b_data(NULL) at jwe.c:463 -> SIGSEGV

A single HTTP request with a crafted Authorization header crashes the
worker process. Trigger token (JOSE header {"alg":"A128KW","enc":"A128GCM"},
empty CEK section between the two dots):

  eyJhbGciOiJBMTI4S1ciLCJlbmMiOiJBMTI4R0NNIn0..AAAAAAAAAAAAAAAA.AA.AA

Reachable in any configuration using the jwt_decrypt_secret converter.
The other two decrypt converters (jwt_decrypt_jwk, jwt_decrypt_cert)
already have the check.

This must be backported as far as JWE support exists.