use bytes_read variable in readinto methods

Author: nwh

fs/ftpfs.py

@@ -239,7 +239,7 @@ def readinto(self, buffer):
         # type: (bytearray) -> int
         data = self.read(len(buffer))
         bytes_read = len(data)
-        buffer[: len(data)] = data
+        buffer[:bytes_read] = data
         return bytes_read
 
     def readline(self, size=-1):

fs/iotools.py

@@ -117,7 +117,7 @@ def readinto(self, b):
         except AttributeError:
             data = self._f.read(len(b))
             bytes_read = len(data)
-            b[: len(data)] = data
+            b[:bytes_read] = data
             return bytes_read
 
     @typing.no_type_check
@@ -128,7 +128,7 @@ def readinto1(self, b):
         except AttributeError:
             data = self._f.read1(len(b))
             bytes_read = len(data)
-            b[: len(data)] = data
+            b[:bytes_read] = data
             return bytes_read
 
     def readline(self, limit=-1):