Silence -Wcomma warning

We use a comma-in-paren expression to evaluate multiple statements
in a condition.  Clang warns that this may be incorrect.  Follow
its suggestion to cast all but the last expression to `void` to
silence the warning.
Status Job ID Name Coverage
  External
passed cdash

passed cmake-dash3win7-windows-release-ninja

passed cmake-hythloth-linux-debug-makefiles+sphinx

passed cmake-hythloth-linux-release-lint-iwyu-tidy

passed cmake-moonshade-osx-debug-ninja

passed cmake-moonshade-osx-release-makefiles

passed cmake-terfin-windows-release-vs2017

passed ghostflow-check-master

passed ghostflow-stager

passed cmake-dash3win7-windows-release-ninja

passed cmake-dash3win7-windows-release-ninja

passed cmake-dash3win7-windows-release-ninja

00:24:45

passed cmake-dash3win7-windows-release-ninja

passed cmake-hythloth-linux-debug-makefiles+sphinx

passed cmake-hythloth-linux-debug-makefiles+sphinx

00:53:06

passed cmake-hythloth-linux-debug-makefiles+sphinx

passed cmake-hythloth-linux-debug-makefiles+sphinx

passed cmake-hythloth-linux-release-lint-iwyu-tidy

passed cmake-hythloth-linux-release-lint-iwyu-tidy

00:02:08

passed cmake-hythloth-linux-release-lint-iwyu-tidy

passed cmake-hythloth-linux-release-lint-iwyu-tidy

passed cmake-moonshade-osx-debug-ninja

passed cmake-moonshade-osx-debug-ninja

passed cmake-moonshade-osx-debug-ninja

passed cmake-moonshade-osx-debug-ninja

00:16:05

passed cmake-moonshade-osx-release-makefiles

passed cmake-moonshade-osx-release-makefiles

passed cmake-moonshade-osx-release-makefiles

passed cmake-moonshade-osx-release-makefiles

00:12:39

passed cmake-terfin-windows-release-vs2017

01:07:52

passed cmake-terfin-windows-release-vs2017

passed cmake-terfin-windows-release-vs2017

passed cmake-terfin-windows-release-vs2017

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager