Fix for incorrect locking in GetPubKey() (keystore.cpp)

zcash: cherry picked from commit 220f950ab1302f8f8c0bb5b9c818f3d6569796bd
zcash: https://github.com/bitcoin/bitcoin/pull/7913

src/keystore.cpp

@@ -18,6 +18,7 @@ bool CBasicKeyStore::GetPubKey(const CKeyID &address, CPubKey &vchPubKeyOut) con
 {
     CKey key;
     if (!GetKey(address, key)) {
+        LOCK(cs_KeyStore);
         WatchKeyMap::const_iterator it = mapWatchKeys.find(address);
         if (it != mapWatchKeys.end()) {
             vchPubKeyOut = it->second;