Better handle invalid parameters to signrawtransaction

This silently skips trying to merge signatures from inputs which
do not exist from transactions provided to signrawtransaction,
instead of hitting an assert.

src/rpc/rawtransaction.cpp

@@ -828,7 +828,9 @@ UniValue signrawtransaction(const UniValue& params, bool fHelp)
 
         // ... and merge in other signatures:
         BOOST_FOREACH(const CMutableTransaction& txv, txVariants) {
-            sigdata = CombineSignatures(prevPubKey, TransactionSignatureChecker(&txConst, i, amount), sigdata, DataFromTransaction(txv, i));
+            if (txv.vin.size() > i) {
+                sigdata = CombineSignatures(prevPubKey, TransactionSignatureChecker(&txConst, i, amount), sigdata, DataFromTransaction(txv, i));
+            }
         }
 
         UpdateTransaction(mergedTx, i, sigdata);