Fix finally exception chaining on recursion

In this case zend_exception_set_previous() would destroy the
fast_call exception and further accesses on ex would be invalid.
We should only update ex if we update EG(exception).

Fixes oss-fuzz #40464.

Zend/tests/try/try_finally_recursive_previous.phpt

@@ -0,0 +1,18 @@
+--TEST--
+Test case where the implicit previous finally exception would result in recursion
+--FILE--
+<?php
+try {
+    $e = new Exception("M1");
+    try {
+        throw new Exception("M2", 0, $e);
+    } finally {
+        throw $e;
+    }
+} finally {}
+?>
+--EXPECTF--
+Fatal error: Uncaught Exception: M1 in %s:%d
+Stack trace:
+#0 {main}
+  thrown in %s on line %d

Zend/zend_vm_def.h

@@ -7714,9 +7714,8 @@ ZEND_VM_HELPER(zend_dispatch_try_catch_finally_helper, ANY, ANY, uint32_t try_ca
 				if (ex) {
 					zend_exception_set_previous(ex, Z_OBJ_P(fast_call));
 				} else {
-					EG(exception) = Z_OBJ_P(fast_call);
+					ex = EG(exception) = Z_OBJ_P(fast_call);
 				}
-				ex = Z_OBJ_P(fast_call);
 			}
 		}
 	}

Zend/zend_vm_execute.h

@@ -2905,9 +2905,8 @@ static zend_never_inline ZEND_OPCODE_HANDLER_RET ZEND_FASTCALL zend_dispatch_try
 				if (ex) {
 					zend_exception_set_previous(ex, Z_OBJ_P(fast_call));
 				} else {
-					EG(exception) = Z_OBJ_P(fast_call);
+					ex = EG(exception) = Z_OBJ_P(fast_call);
 				}
-				ex = Z_OBJ_P(fast_call);
 			}
 		}
 	}