ping: Fix signed 64-bit integer overflow in RTT calculation #762

Job	Run time
job (debian:stable, gcc, i386, 1)	45s
job (opensuse/leap, clang)	1m 46s
job (debian:testing, arm64, aarch64-linux-gnu-gcc, cross-compile)	46s
job (debian:stable, gcc, -DUSE_CAP=false -DUSE_IDN=false -DBUILD_ARPING=false -DBUILD_CLOCKDIFF=f...	38s
job (alpine:latest, gcc, -DBUILD_HTML_MANS=false -DBUILD_MANS=false)	21s
job (ubuntu:jammy, gcc)	42s
job (debian:testing, s390x, s390x-linux-gnu-gcc, cross-compile)	49s
job (ubuntu:bionic, clang, -DUSE_GETTEXT=false, true, true)	39s
job (debian:testing, ppc64el, powerpc64le-linux-gnu-gcc, cross-compile)	52s
job (quay.io/centos/centos:stream9, gcc)	1m 36s
job (rockylinux:9, gcc)	1m 6s
job (rockylinux:8, gcc)	1m 27s
job (fedora:latest, clang)	39s
job (debian:oldstable, clang, true)	36s
job (debian:testing, gcc, -DNO_SETCAP_OR_SUID=false)	44s
job (debian:stable, gcc)	37s
	14m 3s

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