pgsql: Fix dereference in a couple of GUC check hooks

Fix dereference in a couple of GUC check hooks

check_backtrace_functions() and check_archive_directory() were doing an
empty-string check this way:
    *newval[0] == '\0'
which, because of operator precedence, is interpreted as *(newval[0])
instead of (*newval)[0] -- but these variables are pointers to C-strings
and we want to check the first character therein, rather than check the
first pointer of the array, so that interpretation is wrong.  This would
be wrong for any index element other than 0, as evidenced by every other
dereference of the same variable in check_backtrace_functions, which use
parentheses.

Add parentheses to make the intended dereference explicit.

This is just cosmetic at this stage, so no backpatch, although it's been
"wrong" for a long time.

Author: Zhang Hu <kongbaik228@gmail.com>
Reviewed-by: Junwang Zhao <zhjwpku@gmail.com>
Reviewed-by: Chao Li <lic@highgo.com>
Discussion: https://postgr.es/m/CAB5m2QssN6UO+ckr6ZCcV0A71mKUB6WdiTw1nHo43v4DTW1Dfg@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/5f2350a043db08ab0ea48d22f51acfa5d9b374d9

Modified Files
--------------
contrib/basic_archive/basic_archive.c | 2 +-
src/backend/utils/error/elog.c        | 2 +-
2 files changed, 2 insertions(+), 2 deletions(-)