sftp: Handle short reads of sftp_async_read().

sftp_async_read() and sftp_async_read_begin() assume that the whole read
will be successful but when this is not the case, the offset will be
wrong.

Signed-off-by: Andreas Schneider <asn@cryptomilk.org>
(cherry picked from commit d1df255df4)

src/sftp.c

@@ -1945,9 +1945,8 @@ int sftp_async_read(sftp_file file, void *data, uint32_t size, uint32_t id){
         return SSH_ERROR;
       }
       len = ssh_string_len(datastring);
-      //handle->offset+=len;
-      /* We already have set the offset previously. All we can do is warn that the expected len
-       * and effective lengths are different */
+      /* Update the offset with the correct value */
+      file->offset = file->offset - (size - len);
       memcpy(data, ssh_string_data(datastring), len);
       ssh_string_free(datastring);
       sftp_leave_function();