ffmpeg_source: do not adjust offset on read error/eof

It has been observed that NuCachedSource2 can return a negative
value on attempts to read at EOS (end of stream). Also, the
url_feof from FFMPEG attempts an additional fill_buffer after
a eof_reached condition has been set. If the offset is adjusted
backwards at EOS, then url_feof will never detect the EOS and
will have an infinite stream of data. This is particularly bad
when memory is allocated on each line of data read (eg. when
an hls,applehttp media type is parsed via hls_read_header).

Change-Id: Ieb992e2e9e2a55adaabc28fb407f519e56b10011