TPAC: Fix series display for multiple series fields

TPAC: Fix series display for multiple series fields

This commit adds one test record, "Harry Potter and the Goblet of Fire",
which contains multiple series fields. TPAC would display the series
fields in a purely additive fashion as the variable was not reset after
each field. Now we initialize the series var for each new series field
we retrieve, fixing the display problem.

Signed-off-by: Dan Scott <dan@coffeecode.net>
Signed-off-by: Ben Shum <bshum@biblio.org>