keystore: speedup for Old_KeyStore

from Electron-Cash/Electron-Cash@01177d7deb1a56b423af6cf73a9b8ff5d64b24b3

electrum/keystore.py

@@ -451,14 +451,15 @@ class Old_KeyStore(Deterministic_KeyStore):
 
     def get_private_key(self, sequence, password):
         seed = self.get_hex_seed(password)
-        self.check_seed(seed)
-        for_change, n = sequence
         secexp = self.stretch_key(seed)
+        self.check_seed(seed, secexp=secexp)
+        for_change, n = sequence
         pk = self.get_private_key_from_stretched_exponent(for_change, n, secexp)
         return pk, False
 
-    def check_seed(self, seed):
-        secexp = self.stretch_key(seed)
+    def check_seed(self, seed, *, secexp=None):
+        if secexp is None:
+            secexp = self.stretch_key(seed)
         master_private_key = ecc.ECPrivkey.from_secret_scalar(secexp)
         master_public_key = master_private_key.get_public_key_bytes(compressed=False)[1:]
         if master_public_key != bfh(self.mpk):