PyLong_FromString(): fix Coverity CID 1424951

[vstinner]

Explicitly cast digits (Py_ssize_t) to double to fix the following
false-alarm warning from Coverity:

"fsize_z = digits * log_base_BASE[base] + 1;"

CID 1424951: Incorrect expression (UNINTENDED_INTEGER_DIVISION)
Dividing integer expressions "9223372036854775783UL" and "4UL", and
then converting the integer quotient to type "double". Any remainder,
or fractional part of the quotient, is ignored.

[mdickinson]

Fix LGTM, but I'm not particularly happy about the use of double arithmetic here. Surely we can come up with an integer-only solution to detecting overflow?

But that's orthogonal to this PR.

[vstinner]

Fix LGTM, but I'm not particularly happy about the use of double arithmetic here. Surely we can come up with an integer-only solution to detecting overflow?

Honestly, I was very surprised to see the usage of double in PyLong_FromString()!

But I don't feel able to implement logarithm in fixed precision arithmetic or something like that, to replace the current code. If we overestimate the size of the integer, we can trigger an error whereas it was technically possible to create the integer.

I recall vaguely that we use simple arithmetic on integers to estimate the size of an integer (estimate logarithm only with integers) in another Python function, but I don't recall where, sorry :-)

cc @serhiy-storchaka: I recall vaguely that you increased the precison of this estimation. Do you recall where it was?