Update xhprof.c

php-5.5.8 Segmentation fault
details:
(gdb) f 0
#0  hp_execute_internal (execute_data=0x7fffffffa600, fci=0x7fffffffa780, ret=1) at /opt/xhprof/extension/xhprof.c:1708
1708	    temp_variable *retvar = &EX_T(opline->result.var);
(gdb) 
#0  hp_execute_internal (execute_data=0x7fffffffa600, fci=0x7fffffffa780, ret=1) at /opt/xhprof/extension/xhprof.c:1708
1708	    temp_variable *retvar = &EX_T(opline->result.var);
(gdb) print opline
$1 = (zend_op *) 0x0