whitelist: fix SECP256K1_WHITELIST_MAX_N_KEYS constant

"MAX" should mean inclusive. And the whitelisting functions handled this
inconsistently.

Author: jonasnick

include/secp256k1_whitelist.h

@@ -13,7 +13,7 @@
 extern "C" {
 #endif
 
-#define SECP256K1_WHITELIST_MAX_N_KEYS	256
+#define SECP256K1_WHITELIST_MAX_N_KEYS 255
 
 /** Opaque data structure that holds a parsed whitelist proof
  *

src/modules/whitelist/main_impl.h

@@ -144,7 +144,7 @@ int secp256k1_whitelist_signature_parse(const secp256k1_context* ctx, secp256k1_
     }
 
     sig->n_keys = input[0];
-    if (sig->n_keys >= MAX_KEYS || input_len != 1 + 32 * (sig->n_keys + 1)) {
+    if (sig->n_keys > MAX_KEYS || input_len != 1 + 32 * (sig->n_keys + 1)) {
         return 0;
     }
     memcpy(&sig->data[0], &input[1], 32 * (sig->n_keys + 1));