Fix segmentation fault in debug_backtrace()

This pull request fixes a segmentation fault in `debug_backtrace()` by adding a NULL-check for `call->func`. Here's a bit of context from Zend/zend_builtin_functions.c

```c
object = call ? call->object : NULL;
if (object &&
    call->func &&                                      /* <-- This is the added line */
    call->func->type == ZEND_INTERNAL_FUNCTION &&
    !call->func->common.scope) {
        object = NULL;
}

if (call && call->func) {                              /* This is what it's consistent with */
        // ...
} else {
        func = NULL;
        function_name = NULL;
}
```