don't try to decode invalid encoded ext keys

src/base58.h

@@ -146,7 +146,10 @@ public:
 
     K GetKey() {
         K ret;
+        if (vchData.size() == Size) {
+            //if base58 encouded data not holds a ext key, return a !IsValid() key
             ret.Decode(&vchData[0]);
+        }
         return ret;
     }