Fix segmentation faults with enabled keep_descriptor_pool_after_request (#16993)

(cherry picked from commit e844510ee3)

php/ext/google/protobuf/protobuf.c

@@ -143,11 +143,11 @@ static PHP_GINIT_FUNCTION(protobuf) { protobuf_globals->global_symtab = NULL; }
 static PHP_RINIT_FUNCTION(protobuf) {
   // Create the global generated pool.
   // Reuse the symtab (if any) left to us by the last request.
-  upb_DefPool* symtab = PROTOBUF_G(global_symtab);
-  if (!symtab) {
-    PROTOBUF_G(global_symtab) = symtab = upb_DefPool_New();
-    zend_hash_init(&PROTOBUF_G(name_msg_cache), 64, NULL, NULL, 0);
-    zend_hash_init(&PROTOBUF_G(name_enum_cache), 64, NULL, NULL, 0);
+  if (!PROTOBUF_G(global_symtab)) {
+    zend_bool persistent = PROTOBUF_G(keep_descriptor_pool_after_request);
+    PROTOBUF_G(global_symtab) = upb_DefPool_New();
+    zend_hash_init(&PROTOBUF_G(name_msg_cache), 64, NULL, NULL, persistent);
+    zend_hash_init(&PROTOBUF_G(name_enum_cache), 64, NULL, NULL, persistent);
   }
 
   zend_hash_init(&PROTOBUF_G(object_cache), 64, NULL, NULL, 0);
@@ -308,7 +308,7 @@ zend_module_entry protobuf_module_entry = {
     protobuf_functions,            // function list
     PHP_MINIT(protobuf),           // process startup
     PHP_MSHUTDOWN(protobuf),       // process shutdown
-    PHP_RINIT(protobuf),           // request shutdown
+    PHP_RINIT(protobuf),           // request startup
     PHP_RSHUTDOWN(protobuf),       // request shutdown
     NULL,                          // extension info
     PHP_PROTOBUF_VERSION,          // extension version