glsl: Fix NULL pointer dereferences when linking fails.

glsl: Fix NULL pointer dereferences when linking fails.

Commit 7cfefe6965d50 introduced a check for whether linked->Type equals
GL_GEOMETRY_SHADER.  However, linked may be NULL due to an earlier error
condition.

Since the entire function after the error path is (or should be) guarded
by linked != NULL checks, we may as well just return early and remove
the checks.

Fixes crashes in 9 Piglit tests.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Paul Berry <stereotype441@gmail.com>