glsl/nir: return failure from link_varyings if there is a linker error

Some linker errors are set very deep inside link_varyings. Don't return
a success if we can't continue.

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/26310>

src/compiler/glsl/gl_nir_link_varyings.c

@@ -4390,7 +4390,7 @@ link_varyings(struct gl_shader_program *prog, unsigned first,
                              has_xfb_qualifiers, mem_ctx))
       return false;
 
-   return true;
+   return prog->data->LinkStatus != LINKING_FAILURE;
 }
 
 /**