Update in check_max_stack_depth

Hi all,
when I was testing the work in low-memory conditions, I setted ulimit -s
512 and received an error message:

LOG: invalid value for parameter "max_stack_depth": 100
DETAIL: "max_stack_depth" must not exceed 0kB.
HINT: Increase the platform's stack depth limit via "ulimit -s" or local
equivalent.
FATAL: failed to initialize max_stack_depth to 100

With lower stack values, the figure was negative

DETAIL: "max_stack_depth" must not exceed -256kB at ulimit -s 256

It appears because stack_rlimit is less than or equal to STACK_DEPTH_SLOP,
and I added a check. Could you take a look?

Best regards,
Daria Shanina

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С уважением,
Шанина Дарья Александровна