Verify compressed argument in secp256k1_eckey_pubkey_serialize

Due to similarity to the public API function `secp256k1_ec_pubkey_serialize`,
public API flags like `SECP256K1_EC_COMPRESSED` are sometimes mistakingly
passed to newly proposed code (this is currently the case for several modules in
secp256k1-zkp, see BlockstreamResearch/secp256k1-zkp#300).
which is currently not detected. To avoid this in the future, a VERIFY_CHECK
is added to check that the `compressed` argument is either 0 or 1.

src/eckey_impl.h

@@ -9,6 +9,7 @@
 
 #include "eckey.h"
 
+#include "util.h"
 #include "scalar.h"
 #include "field.h"
 #include "group.h"
@@ -35,6 +36,8 @@ static int secp256k1_eckey_pubkey_parse(secp256k1_ge *elem, const unsigned char
 }
 
 static int secp256k1_eckey_pubkey_serialize(secp256k1_ge *elem, unsigned char *pub, size_t *size, int compressed) {
+    VERIFY_CHECK(compressed == 0 || compressed == 1);
+
     if (secp256k1_ge_is_infinity(elem)) {
         return 0;
     }

src/secp256k1.c

@@ -280,7 +280,7 @@ int secp256k1_ec_pubkey_serialize(const secp256k1_context* ctx, unsigned char *o
     ARG_CHECK(pubkey != NULL);
     ARG_CHECK((flags & SECP256K1_FLAGS_TYPE_MASK) == SECP256K1_FLAGS_TYPE_COMPRESSION);
     if (secp256k1_pubkey_load(ctx, &Q, pubkey)) {
-        ret = secp256k1_eckey_pubkey_serialize(&Q, output, &len, flags & SECP256K1_FLAGS_BIT_COMPRESSION);
+        ret = secp256k1_eckey_pubkey_serialize(&Q, output, &len, !!(flags & SECP256K1_FLAGS_BIT_COMPRESSION));
         if (ret) {
             *outputlen = len;
         }