Fix use of uninitialized variable in inline_function().

Fix use of uninitialized variable in inline_function().

Commit e717a9a18 introduced a code path that bypassed the call of
get_expr_result_type, which is not good because we need its rettupdesc
result to pass to check_sql_fn_retval.  We'd failed to notice right
away because the code path in which check_sql_fn_retval uses that
argument is fairly hard to reach in this context.  It's not impossible
though, and in any case inline_function would have no business
assuming that check_sql_fn_retval doesn't need that value.

To fix, move get_expr_result_type out of the if-block, which in
turn requires moving the construction of the dummy FuncExpr
out of it.

Per report from Ranier Vilela.  (I'm bemused by the lack of any
compiler complaints...)

Discussion: https://postgr.es/m/CAEudQAqBqQpQ3HruWAGU_7WaMJ7tntpk0T8k_dVtNB46DqdBgw@mail.gmail.com