Re: Converting tab-complete.c's else-if chain to a switch

Hi,

bd1276a3c9 seems to have introduced a segfault when trying to complete
a word that doesn't have any match. For example, 'postgres=# z\t' will
yield the following backtrace:

#0: psql`pg_strcasecmp(s1="", s2="ACCESS METHOD") at pgstrcasecmp.c:40:39
#1: psql`psql_completion(text=":pgss-", start=0, end=6) at
tab-complete.in.c:2033:8

The matching code in tab-complete.in.c:
if (matches == NULL)
{
const pgsql_thing_t *wac;
for (wac = words_after_create; wac->name != NULL; wac++)
{
if (pg_strcasecmp(prev_wd, wac->name) == 0)
{

psql tries to read the previous word when matches are NULL. However,
there's no previous word set here, leading to the segfault.

I've attached a patch that checks if the previous word does exist
before trying to use it, along with an additional test for this
codepath.