JNI: fix use sig array

Author: fingera

src/java/org_bitcoin_NativeSecp256k1.c

@@ -81,15 +81,15 @@ SECP256K1_API jobjectArray JNICALL Java_org_bitcoin_NativeSecp256k1_secp256k1_1e
   jbyteArray sigArray, intsByteArray;
   unsigned char intsarray[2];
 
-  secp256k1_ecdsa_signature sig[72];
+  secp256k1_ecdsa_signature sig;
 
-  int ret = secp256k1_ecdsa_sign(ctx, sig, data, secKey, NULL, NULL);
+  int ret = secp256k1_ecdsa_sign(ctx, &sig, data, secKey, NULL, NULL);
 
   unsigned char outputSer[72];
   size_t outputLen = 72;
 
   if( ret ) {
-    int ret2 = secp256k1_ecdsa_signature_serialize_der(ctx,outputSer, &outputLen, sig ); (void)ret2;
+    int ret2 = secp256k1_ecdsa_signature_serialize_der(ctx,outputSer, &outputLen, &sig ); (void)ret2;
   }
 
   intsarray[0] = outputLen;