Add some missing `VERIFY_CHECK(ctx != NULL)` for functions that use `ARG_CHECK`

The `ARG_CHECK` macro requires that a variable called `ctx` exist and be
non-NULL. However, in several functions that do not use the context variable,
we simply ignore it with `(void)ctx`. Replace these with explicit checks for
non-NULLness to avoid invalid memory accesses.

src/secp256k1.c

@@ -150,7 +150,6 @@ static void secp256k1_pubkey_save(secp256k1_pubkey* pubkey, secp256k1_ge* ge) {
 int secp256k1_ec_pubkey_parse(const secp256k1_context* ctx, secp256k1_pubkey* pubkey, const unsigned char *input, size_t inputlen) {
     secp256k1_ge Q;
 
-    (void)ctx;
     VERIFY_CHECK(ctx != NULL);
     ARG_CHECK(pubkey != NULL);
     memset(pubkey, 0, sizeof(*pubkey));
@@ -168,7 +167,6 @@ int secp256k1_ec_pubkey_serialize(const secp256k1_context* ctx, unsigned char *o
     size_t len;
     int ret = 0;
 
-    (void)ctx;
     VERIFY_CHECK(ctx != NULL);
     ARG_CHECK(outputlen != NULL);
     ARG_CHECK(*outputlen >= ((flags & SECP256K1_FLAGS_BIT_COMPRESSION) ? 33 : 65));
@@ -214,7 +212,7 @@ static void secp256k1_ecdsa_signature_save(secp256k1_ecdsa_signature* sig, const
 int secp256k1_ecdsa_signature_parse_der(const secp256k1_context* ctx, secp256k1_ecdsa_signature* sig, const unsigned char *input, size_t inputlen) {
     secp256k1_scalar r, s;
 
-    (void)ctx;
+    VERIFY_CHECK(ctx != NULL);
     ARG_CHECK(sig != NULL);
     ARG_CHECK(input != NULL);
 
@@ -232,7 +230,7 @@ int secp256k1_ecdsa_signature_parse_compact(const secp256k1_context* ctx, secp25
     int ret = 1;
     int overflow = 0;
 
-    (void)ctx;
+    VERIFY_CHECK(ctx != NULL);
     ARG_CHECK(sig != NULL);
     ARG_CHECK(input64 != NULL);
 
@@ -251,7 +249,7 @@ int secp256k1_ecdsa_signature_parse_compact(const secp256k1_context* ctx, secp25
 int secp256k1_ecdsa_signature_serialize_der(const secp256k1_context* ctx, unsigned char *output, size_t *outputlen, const secp256k1_ecdsa_signature* sig) {
     secp256k1_scalar r, s;
 
-    (void)ctx;
+    VERIFY_CHECK(ctx != NULL);
     ARG_CHECK(output != NULL);
     ARG_CHECK(outputlen != NULL);
     ARG_CHECK(sig != NULL);
@@ -263,7 +261,7 @@ int secp256k1_ecdsa_signature_serialize_der(const secp256k1_context* ctx, unsign
 int secp256k1_ecdsa_signature_serialize_compact(const secp256k1_context* ctx, unsigned char *output64, const secp256k1_ecdsa_signature* sig) {
     secp256k1_scalar r, s;
 
-    (void)ctx;
+    VERIFY_CHECK(ctx != NULL);
     ARG_CHECK(output64 != NULL);
     ARG_CHECK(sig != NULL);
 
@@ -396,7 +394,6 @@ int secp256k1_ec_seckey_verify(const secp256k1_context* ctx, const unsigned char
     int overflow;
     VERIFY_CHECK(ctx != NULL);
     ARG_CHECK(seckey != NULL);
-    (void)ctx;
 
     secp256k1_scalar_set_b32(&sec, seckey, &overflow);
     ret = !overflow && !secp256k1_scalar_is_zero(&sec);
@@ -435,7 +432,6 @@ int secp256k1_ec_privkey_tweak_add(const secp256k1_context* ctx, unsigned char *
     VERIFY_CHECK(ctx != NULL);
     ARG_CHECK(seckey != NULL);
     ARG_CHECK(tweak != NULL);
-    (void)ctx;
 
     secp256k1_scalar_set_b32(&term, tweak, &overflow);
     secp256k1_scalar_set_b32(&sec, seckey, NULL);
@@ -483,7 +479,6 @@ int secp256k1_ec_privkey_tweak_mul(const secp256k1_context* ctx, unsigned char *
     VERIFY_CHECK(ctx != NULL);
     ARG_CHECK(seckey != NULL);
     ARG_CHECK(tweak != NULL);
-    (void)ctx;
 
     secp256k1_scalar_set_b32(&factor, tweak, &overflow);
     secp256k1_scalar_set_b32(&sec, seckey, NULL);