fix: import_address() for existing Imported_Wallet was willing to import anything

lib/wallet.py

@@ -1455,6 +1455,8 @@ class Imported_Wallet(Simple_Wallet):
         return []
 
     def import_address(self, address):
+        if not bitcoin.is_address(address):
+            return ''
         if address in self.addresses:
             return ''
         self.addresses[address] = {}