glsl: avoid leaking linked gl_shader when there's a late linker error

glsl: avoid leaking linked gl_shader when there's a late linker error

This makes piglit mixing-clip-distance-and-clip-vertex-disallowed have 0
definitely lost blocks with valgrind. (Same non-0 number of possibly
lost blocks though.)

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Tobias Klausmann <tobias.johannes.klausmann@mni.thm.de>
Cc: "10.5 10.6" <mesa-stable@lists.freedesktop.org>