GCC-11 sniffed out a long-standing bug where a pointer was being tested for a
negative value, which is impossible. Instead, check for NULL, which is how the
error result is actually returned.
See https://github.com/iwamatsu/slim/issues/14 for details.
Fixes <https://issues.guix.gnu.org/63155>.
* gnu/packages/display-managers.scm (slim) [fix-0-pointer-comparison]: new
phase
Signed-off-by: Ludovic Courtès <ludo@gnu.org>