Fix potential memory clobber in tsvector_concat().

Fix potential memory clobber in tsvector_concat().

tsvector_concat() allocated its result workspace using the "conservative"
estimate of the sum of the two input tsvectors' sizes.  Unfortunately that
wasn't so conservative as all that, because it supposed that the number of
pad bytes required could not grow.  Which it can, as per test case from
Jesper Krogh, if there's a mix of lexemes with positions and lexemes
without them in the input data.  The fix is to assume that we might add
a not-previously-present pad byte for each and every lexeme in the two
inputs; which really is conservative, but it doesn't seem worthwhile to
try to be more precise.

This is an aboriginal bug in tsvector_concat, so back-patch to all
versions containing it.