glsl2: Fixed cloning of ir_call error instructions.

Those have the callee field set to the null pointer, so
calling the public constructor will segfault.

Signed-off-by: Tilman Sauerbeck <tilman@code-monkey.de>

src/glsl/ir_clone.cpp

@@ -141,6 +141,9 @@ ir_loop::clone(void *mem_ctx, struct hash_table *ht) const
 ir_call *
 ir_call::clone(void *mem_ctx, struct hash_table *ht) const
 {
+   if (this->type == glsl_type::error_type)
+      return ir_call::get_error_instruction(mem_ctx);
+
    exec_list new_parameters;
 
    foreach_iter(exec_list_iterator, iter, this->actual_parameters) {