Clamp walletpassphrase timeout to 2^(30) seconds and check its bounds

Clamps the timeout of walletpassphrase to 2^(30) seconds, which is
~34 years. Any number greater than that will be forced to be
2^(30). This avoids the sign flipping problem with large values which
can result in a negative time used.

Also perform bounds checks to ensure that the timeout is positive
to avoid immediate relocking of the wallet.

src/wallet/rpcwallet.cpp

@@ -2269,7 +2269,8 @@ UniValue walletpassphrase(const JSONRPCRequest& request)
             "This is needed prior to performing transactions related to private keys such as sending bitcoins\n"
             "\nArguments:\n"
             "1. \"passphrase\"     (string, required) The wallet passphrase\n"
-            "2. timeout            (numeric, required) The time to keep the decryption key in seconds.\n"
+            "2. timeout            (numeric, required) The time to keep the decryption key in seconds. Limited to at most 1073741824 (2^30) seconds.\n"
+            "                                          Any value greater than 1073741824 seconds will be set to 1073741824 seconds.\n"
             "\nNote:\n"
             "Issuing the walletpassphrase command while the wallet is already unlocked will set a new unlock\n"
             "time that overrides the old one.\n"
@@ -2298,6 +2299,17 @@ UniValue walletpassphrase(const JSONRPCRequest& request)
     // Alternately, find a way to make request.params[0] mlock()'d to begin with.
     strWalletPass = request.params[0].get_str().c_str();
 
+    // Get the timeout
+    int64_t nSleepTime = request.params[1].get_int64();
+    // Timeout cannot be negative, otherwise it will relock immediately
+    if (nSleepTime < 0) {
+        throw JSONRPCError(RPC_INVALID_PARAMETER, "Timeout cannot be negative.");
+    }
+    // Clamp timeout to 2^30 seconds
+    if (nSleepTime > (int64_t)1 << 30) {
+        nSleepTime = (int64_t)1 << 30;
+    }
+
     if (strWalletPass.length() > 0)
     {
         if (!pwallet->Unlock(strWalletPass)) {
@@ -2311,7 +2323,6 @@ UniValue walletpassphrase(const JSONRPCRequest& request)
 
     pwallet->TopUpKeyPool();
 
-    int64_t nSleepTime = request.params[1].get_int64();
     pwallet->nRelockTime = GetTime() + nSleepTime;
     RPCRunLater(strprintf("lockwallet(%s)", pwallet->GetName()), boost::bind(LockWallet, pwallet), nSleepTime);