Makes sure it is consistent for a later commit when action exection is more fragmented.

Avoids unnecessary pointer parameter.

Providing an out-of-range index caused an assert panic.

Panic was:

Panic: file mail-index-transaction-update.c: line 19 (mail_index_transaction_lookup): assertion failed: (seq >= t->first_new_seq && seq <= t->last_new_seq)

testsuite: testsuite-mailstore - Return struct testsuite_mailstore_mail from testsuite_mailstore_open().

This is more flexible and needed in subsequent commit.