scripts: mk-catalog.sh: make sure readlink invocation never fail
scripts: mk-catalog.sh: make sure readlink invocation never fail

This fixes the case when glslsandbox-player is in the PATH.

Signed-off-by: Julien Olivain <7fdfe229fce69a4d7f38653f3755ccbae5703f88@cotds.org>

jolivain/glslsandbox-player,jolivain/glslsandbox-player,jolivain/glslsandbox-player,jolivain/glslsandbox-player