Extend try catch block around calls to libsnark, per discussion in #1126.

src/zcash/JoinSplit.cpp

@@ -139,6 +139,7 @@ public:
             throw std::runtime_error("JoinSplit verifying key not loaded");
         }
 
+        try {
             r1cs_ppzksnark_proof<ppzksnark_ppT> r1cs_proof;
             std::stringstream ss;
             std::string proof_str(proof.begin(), proof.end());
@@ -157,7 +158,6 @@ public:
                 vpub_new
             );
 
-        try {
             return r1cs_ppzksnark_verifier_strong_IC<ppzksnark_ppT>(*vk, witness, r1cs_proof);
         } catch (...) {
             return false;