Description:
------------
In all cases (but the one below one), "file" and "line" indexes are defined in each frame of the output of debug_backtrace.

But when using call_user_function() or call_user_function_array, "file" and "line" is not defined for one of the frames.

It would be nice if I could count on file and line always being defined when using the output of debug_backtrace, thereby avoiding code for special cases.

For the missing file and line in the example code below, when calling f() inside call_user_function(), I suggest using the file and line of the call_user_function() call.

See also bug 38047 and 24405.

Reproduce code:
---------------
<?php
error_reporting(E_ALL | E_STRICT);


function f() {
  $a = $b;
  var_dump(debug_backtrace());
}

//The backtrace generated here will not have file and line defined
call_user_func("f", Array());

//The backtrace generated here will have file and line defined.
f();

?>

Expected result:
----------------
The frame with index 0 of the first debug_backtrace_call should contain indexes "file" => "test.php" and "line" => 6

Actual result:
--------------
array(2) {
  [0]=>
  array(2) {
    ["function"]=>
    string(1) "f"
    ["args"]=>
    array(1) {
      [0]=>
      &array(0) {
      }
    }
  }
  [1]=>
  array(4) {
    ["file"]=>
    string(18) "/home/tjk/test.php"
    ["line"]=>
    int(6)
    ["function"]=>
    string(14) "call_user_func"
    ["args"]=>
    array(2) {
      [0]=>
      &string(1) "f"
      [1]=>
      &array(0) {
      }
    }
  }
}
array(1) {
  [0]=>
  array(4) {
    ["file"]=>
    string(18) "/home/tjk/test.php"
    ["line"]=>
    int(8)
    ["function"]=>
    string(1) "f"
    ["args"]=>
    array(0) {
    }
  }
}