gui: fix compiler warning in formatPingTime() #18252

With Clang 10.0.0:

qt/guiutil.cpp:778:26: warning: implicit conversion from 'std::__1::numeric_limits<long>::type'
      (aka 'long') to 'double' changes value from 9223372036854775807 to 9223372036854775808
      [-Wimplicit-int-float-conversion]
    return (dPingTime == std::numeric_limits<int64_t>::max()/1e6 || dPingTime == 0) ? QObject::...
                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

When assigning to dPingTime a similar typecast is used:

src/net.cpp:558:    stats.dPingTime = (((double)nPingUsecTime) / 1e6);

so it should be ok to change 9223372036854775807 to 9223372036854775808.

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Here's a proposed alternative - representing these values as int64_t internally, and only converting to double at the time of display. Motivated by a general distrust of floating point and a desire to minimize the points at which it's used: https://github.com/bitcoin/bitcoin/compare/master...Empact:2020-03-ping-time

Concept ACK on fixing but I prefer @Empact's approach :)

From a quick look and comparison I also prefer @Empact approach.

Opened the alternative as #18260

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Conflicts

Reviewers, this pull request conflicts with the following ones:

• #18260 (refactor: Fix implicit value conversion in formatPingTime by Empact)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

Given the comments here, I'm going to close this in favour of #18260.

Excellent! Yes, #18260 clearly supersedes this PR.