BUG/MEDIUM: jwt: fix heap overflow in ECDSA signature DER conversion

BUG/MEDIUM: jwt: fix heap overflow in ECDSA signature DER conversion

convert_ecdsa_sig() calls i2d_ECDSA_SIG(ecdsa_sig, &p) where p
points into signature->area, a trash chunk of tune.bufsize bytes
(default 16384). i2d writes with no output bound.

The raw R||S input can be up to bufsize bytes (filled by
base64urldec at jwt.c:520-527), giving bignum_len up to 8192. The
DER encoding adds a SEQUENCE header (2-4 bytes), two INTEGER headers
(2-4 bytes each), and up to two leading-zero sign-padding bytes when
the bignum high bit is set. With two 8192-byte bignums having the
high bit set, the encoding is ~16398 bytes, overflowing the 16384-
byte buffer by ~14 bytes.

Triggered by any JWT with alg=ES256/384/512 and a ~21830-character
base64url signature. The signature does not need to verify
successfully; the overflow happens before verification. Reachable
from any config using jwt_verify with an EC algorithm.

Also fixes the existing wrong check: i2d returns -1 on error which
became SIZE_MAX in the size_t signature->data, defeating the
"== 0" test.

This must be backported as far as JWT support exists.