glsl: Guard against NULL dereference

This trivially corrects mesa 3ca1c221, which introduced a check that
crashes when a match is not found.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=95005
Fixes: piglit.spec.glsl-1_50.compiler.interface-blocks-name-reused-globally-4.vert
Reviewed-by: Alejandro PiƱeiro <apinheiro@igalia.com>
1 file changed