Don't return reference from Fiber::getReturn()

Even if the fiber function returns by reference, we must return
a value from Fiber::getReturn() to satisfy the function signature.

Fixes oss-fuzz #36417.

Author: nikic

Zend/tests/fibers/return-by-ref.phpt

@@ -0,0 +1,17 @@
+--TEST--
+Fiber function may return by ref, but getReturn() always returns by val
+--FILE--
+<?php
+
+$fiber = new Fiber(function &() {
+    Fiber::suspend();
+    return $var;
+});
+
+$fiber->start();
+$fiber->resume();
+var_dump($fiber->getReturn());
+
+?>
+--EXPECT--
+NULL

Zend/zend_fibers.c

@@ -832,7 +832,7 @@ ZEND_METHOD(Fiber, getReturn)
 		} else if (fiber->flags & ZEND_FIBER_FLAG_BAILOUT) {
 			message = "The fiber exited with a fatal error";
 		} else {
-			RETURN_COPY(&fiber->result);
+			RETURN_COPY_DEREF(&fiber->result);
 		}
 	} else if (fiber->context.status == ZEND_FIBER_STATUS_INIT) {
 		message = "The fiber has not been started";