gst/typefind/gsttypefindfunctions.c: Make plain/text typefinder more conservative: firstly, check for embedded zeroes...

Original commit message from CVS:
* gst/typefind/gsttypefindfunctions.c:
(utf8_type_find_count_embedded_zeroes),
(utf8_type_find_have_valid_utf8_at_offset), (utf8_type_find):
Make plain/text typefinder more conservative: firstly, check
for embedded zeroes, which are perfectly valid UTF-8 characters,
but also a fairly good sign that something is not a plain text
file; secondly, probe into the middle of the file if possible.
If we can't probe into the middle, limit the probability value
to be returned to TYPE_FIND_POSSIBLE (see #333900).