crypto.ecdsa: improve internal sign_digest routine (#23960)

Author: blackshirt

vlib/crypto/ecdsa/ecdsa.v

@@ -363,14 +363,12 @@ fn sign_digest(key &C.EVP_PKEY, digest []u8) ![]u8 {
 	// siglen was used to store the size of the signature output. When EVP_PKEY_sign
 	// was called with NULL signature buffer, siglen will tell maximum size of signature.
 	siglen := usize(C.EVP_PKEY_size(key))
-	st := C.EVP_PKEY_sign(ctx, 0, &siglen, digest.data, digest.len)
-	if st <= 0 {
-		C.EVP_PKEY_CTX_free(ctx)
-		return error('Get null buffer length on EVP_PKEY_sign')
-	}
 	sig := []u8{len: int(siglen)}
+
+	// calls directly with sign
 	do := C.EVP_PKEY_sign(ctx, sig.data, &siglen, digest.data, digest.len)
 	if do <= 0 {
+		unsafe { sig.free() }
 		C.EVP_PKEY_CTX_free(ctx)
 		return error('EVP_PKEY_sign fails to sign message')
 	}