Avoid overflow hazard when clamping group counts to "long int".

Several places in the planner tried to clamp a double value to fit
in a "long" by doing
	(long) Min(x, (double) LONG_MAX);
This is subtly incorrect, because it casts LONG_MAX to double and
potentially back again.  If long is 64 bits then the double value
is inexact, and the platform might round it up to LONG_MAX+1
resulting in an overflow and an undesirably negative output.

While it's not hard to rewrite the expression into a safe form,
let's put it into a common function to reduce the risk of someone
doing it wrong in future.

In principle this is a bug fix, but since the problem could only
manifest with group count estimates exceeding 2^63, it seems unlikely
that anyone has actually hit this or will do so anytime soon.  We're
fixing it mainly to satisfy fuzzer-type tools.  That being the case,
a HEAD-only fix seems sufficient.

Andrey Lepikhov

Discussion: https://postgr.es/m/ebbc2efb-7ef9-bf2f-1ada-d6ec48f70e58@postgrespro.ru
