baseparse: fix invalid avg_bitrate after reset

gst_base_parse_reset() does not reset data_bytecount to 0, so gst_base_parse_update_bitrates() uses a wrong value to calculate the average bitrate on subsequent pipeline starts. This leads to an excessive amount of "tag" events being pushed. These events include very high "bitrate" values that diminish over time, and are produced until the average bitrate is back to sane values.

Fixes #840 (closed)