Fix leak when setting cyclic previous exception in finally

A curious exception handling pattern found in Symfony's HttpClient.

Author: nikic

Zend/tests/exception_set_previous_leak.phpt

@@ -0,0 +1,20 @@
+--TEST--
+Leak when setting recursive previous exception in finally handling
+--FILE--
+<?php
+
+try {
+    try {
+        throw new Exception("Test");
+    } catch (Exception $e) {
+        throw $e;
+    } finally {
+        throw $e;
+    }
+} catch (Exception $e2) {
+    echo $e2->getMessage(), "\n";
+}
+
+?>
+--EXPECT--
+Test

Zend/zend_exceptions.c

@@ -76,9 +76,15 @@ void zend_exception_set_previous(zend_object *exception, zend_object *add_previo
 	zval  pv, zv, rv;
 	zend_class_entry *base_ce;
 
-	if (exception == add_previous || !add_previous || !exception) {
+	if (!exception || !add_previous) {
 		return;
 	}
+
+	if (exception == add_previous) {
+		OBJ_RELEASE(add_previous);
+		return;
+	}
+
 	ZVAL_OBJ(&pv, add_previous);
 	if (!instanceof_function(Z_OBJCE(pv), zend_ce_throwable)) {
 		zend_error_noreturn(E_CORE_ERROR, "Previous exception must implement Throwable");