glsl: Initialize all fields of ir_variable in constructor
Better be safe, even if we could technically avoid this for some fields.
Uninitialized members gave trouble only with
-O3 -flto and resulted in such errors from valgrind:
==17883== Conditional jump or move depends on uninitialised value(s) ==17883== at 0x68F073E: array_sizing_visitor::visit(ir_variable*) ==17883== by 0x68EDCB4: ir_function_signature::accept(ir_hierarchical_visitor*) ==17883== by 0x68EDB6D: ir_function::accept(ir_hierarchical_visitor*) ==17883== by 0x63B822C: visit_list_elements(ir_hierarchical_visitor*, exec_list*, bool)