Too many calls to callback function in feeds_tamper_feeds_after_parse()

Issue #1047894: Behavior when fields/columns are not in import file: do not clear field, but leave field untouched. reveiled that tamper callback-functions are called too often in feeds_tamper_feeds_after_parse(), with the following results:
- $result is appended when it shouldn't (tampered fields not in the current source).
- the issue 1047894 doesn't work for tampered fields
- error in log: Notice: Undefined index: {field_name} in feeds_tamper_feeds_after_parse() (line 22 of \feeds_tamper\feeds_tamper.module).