Fixes #1762 segfault when miner is interrupted.

Running ./zcash-cli setgenerate false would result in a segfault.
The miner thread's boost::signals2::connection was not disconnected
when the miner thread was interrupted and shutdown.  Subsequently, when
a new block arrived, the UpdateTip callback would still be invoked on
a now invalid object, resulting in a segfault.

src/miner.cpp

@@ -647,11 +647,13 @@ void static BitcoinMiner(CWallet *pwallet)
     }
     catch (const boost::thread_interrupted&)
     {
+        c.disconnect();
         LogPrintf("ZcashMiner terminated\n");
         throw;
     }
     catch (const std::runtime_error &e)
     {
+        c.disconnect();
         LogPrintf("ZcashMiner runtime error: %s\n", e.what());
         return;
     }