Fix #78694: Appending to a variant array causes segfault

`write_dimension` object handlers have to be able to handle `NULL`
`offset`s; for now we simply throw an exception instead of following
the `NULL` pointer.

Author: cmb69

NEWS

@@ -6,6 +6,9 @@ PHP                                                                        NEWS
   . Fixed bug #78656 (Parse errors classified as highest log-level). (Erik
     Lundin)
 
+- COM:
+  . Fixed bug #78694 (Appending to a variant array causes segfault). (cmb)
+
 - Date:
   . Fixed bug #70153 (\DateInterval incorrectly unserialized). (Maksim Iakunin)
 

ext/com_dotnet/com_handlers.c

@@ -127,6 +127,11 @@ static void com_write_dimension(zval *object, zval *offset, zval *value)
 
 	obj = CDNO_FETCH(object);
 
+	if (offset == NULL) {
+		php_com_throw_exception(DISP_E_BADINDEX, "appending to variants is not supported");
+		return;
+	}
+
 	if (V_VT(&obj->v) == VT_DISPATCH) {
 		ZVAL_COPY_VALUE(&args[0], offset);
 		ZVAL_COPY_VALUE(&args[1], value);

ext/com_dotnet/tests/bug78694.phpt

@@ -0,0 +1,19 @@
+--TEST--
+Bug #78694 (Appending to a variant array causes segfault)
+--SKIPIF--
+<?php
+if (!extension_loaded('com_dotnet')) die('skip com_dotnet extension not available');
+?>
+--FILE--
+<?php
+foreach ([new com('WScript.Shell'), new variant([])] as $var) {
+    try {
+        $var[] = 42;
+    } catch (com_exception $ex) {
+        var_dump($ex->getMessage());
+    }
+}
+?>
+--EXPECT--
+string(38) "appending to variants is not supported"
+string(38) "appending to variants is not supported"