GNUInstallDirs: avoid unwanted variable dereference

In some situations, it seems that the variable `0` is defined. In the
case found, it was set to `1`. This makes the detection of the missing
third argument bogus and unnecessarily triggers a warning.

This oversight was introduced in 229b5ee9 (GNUInstallDirs: Add dir
argument to GNUInstallDirs_get_absolute_install_dir, 2020-10-31) prior
to CMake 3.20's release cycle.
31 jobs for !6474 with GNUInstallDirs-variable-named-0 in 21 minutes and 54 seconds (queued for 1 second)
latest detached
Status Name Job ID Coverage
  Build
passed build:centos6-x86_64 #6087184
cmake build docker linux manual

00:03:43

passed build:debian10-iwyu #6087181
cmake build docker linux

00:04:38

passed build:fedora33-ninja #6087185
cmake build docker linux linux-3.17 manual

00:02:37

passed build:fedora33-sphinx #6087183
cmake build docker linux linux-3.17

00:04:33

passed build:fedora33-tidy #6087182
cmake build docker linux linux-3.17

00:06:18

manual build:macos-x86_64-makefiles #6087187
cmake macos shell xcode-12.5 nonconcurrent manual
manual build:macos-x86_64-ninja #6087186
cmake macos shell xcode-12.5 nonconcurrent manual
manual build:windows-vs2019-x64-ninja #6087188
cmake windows shell vs2019 msvc-19.29-16.10 nonconcurrent manual
 
  Test
passed test:fedora33-ninja #6087189
cmake docker linux linux-3.17 x11

00:06:25

created test:macos-x86_64-makefiles #6087191
cmake macos shell xcode-12.5 nonconcurrent
created test:macos-x86_64-ninja #6087190
cmake macos shell xcode-12.5 nonconcurrent
created test:windows-vs2019-x64-ninja #6087192
cmake windows shell vs2019 msvc-19.29-16.10 nonconcurrent
 
  Test Ext
passed test:cuda10.2-nvidia #6087195
cmake cuda-rt docker linux

00:03:38

passed test:debian10-ninja #6087193
cmake build docker linux linux-3.17

00:05:13

passed test:fedora33-makefiles #6087194
cmake build docker linux linux-3.17

00:06:48

passed test:fedora33-ninja-multi #6087196
cmake build docker linux linux-3.17

00:06:25

created test:macos-x86_64-xcode #6087197
cmake macos shell xcode-12.5 concurrent
created test:windows-vs2019-x64 #6087198
cmake windows shell vs2019 msvc-19.29-16.10 concurrent
 
  External
passed cdash

passed ghostflow-check-master

passed ghostflow-check-release

passed ghostflow-stager

passed cdash

passed cdash

passed cdash

passed cdash

passed cdash

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager