Return previous error handler when resetting the error handler

set_error_handler(null) and set_exception_handler(null) now return the
previous error/exception handler instead of just returning bool(true).
This is consistent with the behavior of these functions with non-null
values.

Zend/tests/bug60738.phpt

@@ -3,15 +3,20 @@ Bug #60738 Allow 'set_error_handler' to handle NULL
 --FILE--
 <?php
 
-set_error_handler(function() { echo 'Intercepted error!', "\n"; });
+var_dump(set_error_handler(
+    function() { echo 'Intercepted error!', "\n"; }
+));
 
 trigger_error('Error!');
 
-set_error_handler(null);
+var_dump(set_error_handler(null));
 
 trigger_error('Error!');
 ?>
 --EXPECTF--
+NULL
 Intercepted error!
+object(Closure)#1 (0) {
+}
 
 Notice: Error! in %s on line %d

Zend/tests/bug60738_variation.phpt

@@ -0,0 +1,23 @@
+--TEST--
+Bug #60738 Allow 'set_error_handler' to handle NULL
+--FILE--
+<?php
+
+var_dump(set_exception_handler(
+    function() { echo 'Intercepted exception!', "\n"; }
+));
+
+var_dump(set_exception_handler(null));
+
+throw new Exception('Exception!');
+?>
+--EXPECTF--
+NULL
+object(Closure)#1 (0) {
+}
+
+Fatal error: Uncaught exception 'Exception' with message 'Exception!' in %s:%d
+Stack trace:
+#0 {main}
+  thrown in %s on line %d
+

Zend/zend_builtin_functions.c

@@ -1543,7 +1543,7 @@ ZEND_FUNCTION(set_error_handler)
 	if (Z_TYPE_P(error_handler) == IS_NULL) { /* unset user-defined handler */
 		FREE_ZVAL(EG(user_error_handler));
 		EG(user_error_handler) = NULL;
-		RETURN_TRUE;
+		return;
 	}
 
 	EG(user_error_handler_error_reporting) = (int)error_type;
@@ -1614,7 +1614,7 @@ ZEND_FUNCTION(set_exception_handler)
 	if (Z_TYPE_P(exception_handler) == IS_NULL) { /* unset user-defined handler */
 		FREE_ZVAL(EG(user_exception_handler));
 		EG(user_exception_handler) = NULL;
-		RETURN_TRUE;
+		return;
 	}
 
 	MAKE_COPY_ZVAL(&exception_handler, EG(user_exception_handler))