lib: data-stack - simplify expressions in data_stack_last_buffer_reset Noisy expression used more than once, give it a meaningful name.
lib: data-stack - simplify expressions in data_stack_last_buffer_reset
Noisy expression used more than once, give it a meaningful name.

Signed-off-by: Phil Carmody <e888d2bd6f13f82caa51a37c03d034c76f661ba3@dovecot.fi>

dscho/dovecot,dscho/dovecot,dscho/dovecot,dscho/dovecot,dscho/dovecot