mesa: Add assert to check number of vector elements

The below code crashes when vector_elements <= 0
Fixes Warray-bounds warnings

Signed-off-by: Jan Vesely <jan.vesely@rutgers.edu>
Reviewed-by: Jose Fonseca <jfonseca@vmware.com>

src/mesa/program/ir_to_mesa.cpp

@@ -1449,6 +1449,7 @@ ir_to_mesa_visitor::visit(ir_swizzle *ir)
    ir->val->accept(this);
    src = this->result;
    assert(src.file != PROGRAM_UNDEFINED);
+   assert(ir->type->vector_elements > 0);
 
    for (i = 0; i < 4; i++) {
       if (i < ir->type->vector_elements) {

src/mesa/state_tracker/st_glsl_to_tgsi.cpp

@@ -2051,6 +2051,7 @@ glsl_to_tgsi_visitor::visit(ir_swizzle *ir)
    ir->val->accept(this);
    src = this->result;
    assert(src.file != PROGRAM_UNDEFINED);
+   assert(ir->type->vector_elements > 0);
 
    for (i = 0; i < 4; i++) {
       if (i < ir->type->vector_elements) {