Check arguments passed to secp256k1 methods (#94)

* Check arguments passed to secp256k1 methods

Illegal arguments will trigger an internal callback that prints to stderr and calls abort.
We already check arguments in our JNI and kotlin native code but had missed 2 checks (recid in ecdsaRecover, empty arrays in pubkeyCombine).

* Implement the same "tweak" checks in the native code and JNI code

The native code was missing checks on the "tweak" size (which must be 32 bytes)

jni/c/src/fr_acinq_secp256k1_Secp256k1CFunctions.c

@@ -517,6 +517,7 @@ JNIEXPORT jbyteArray JNICALL Java_fr_acinq_secp256k1_Secp256k1CFunctions_secp256
     if (jpubkeys == NULL) return NULL;
 
     count = (*penv)->GetArrayLength(penv, jpubkeys);
+    CHECKRESULT(count < 1, "pubkey array cannot be empty")
     pubkeys = calloc(count, sizeof(secp256k1_pubkey*));
 
     for(i = 0; i < count; i++) {
@@ -600,6 +601,7 @@ JNIEXPORT jbyteArray JNICALL Java_fr_acinq_secp256k1_Secp256k1CFunctions_secp256
     if (jctx == 0) return NULL;
     if (jsig == NULL) return NULL;
     if (jmsg == NULL) return NULL;
+    CHECKRESULT(recid < 0 || recid > 3, "invalid recovery id");
 
     sigSize = (*penv)->GetArrayLength(penv, jsig);
     int sigFormat = GetSignatureFormat(sigSize);