glsl: In update_max_array_access, fix interface instance check.

glsl: In update_max_array_access, fix interface instance check.

In commit f878d20 (glsl: Update ir_variable::max_ifc_array_access
properly), I accidentally used the wrong kind of check to determine
whether the variable being accessed was an interface instance (I used
var->get_interface_type() != NULL when I should have used
var->is_interface_instance()).  As a result, if an unnamed interface
block contained a struct which contained an array,
update_max_array_access() would mistakenly interpret the struct as a
named interface block and try to dereference a null
var->max_ifc_array_access.

This patch corrects the check, fixing the null dereference.

Fixes piglit test interface-block-struct-nesting.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=70368

Reviewed-by: Matt Turner <mattst88@gmail.com>