ERR_FAIL_NULL check file access

Null in ResourceImporterTexture::_save_stex

(cherry picked from commit b74bc42)

editor/import/resource_importer_texture.cpp

@@ -222,6 +222,7 @@ void ResourceImporterTexture::get_import_options(List<ImportOption> *r_options,
 void ResourceImporterTexture::_save_stex(const Ref<Image> &p_image, const String &p_to_path, int p_compress_mode, float p_lossy_quality, Image::CompressMode p_vram_compression, bool p_mipmaps, int p_texture_flags, bool p_streamable, bool p_detect_3d, bool p_detect_srgb, bool p_force_rgbe, bool p_detect_normal, bool p_force_normal, bool p_force_po2_for_compressed) {
 
 	FileAccess *f = FileAccess::open(p_to_path, FileAccess::WRITE);
+	ERR_FAIL_NULL(f);
 	f->store_8('G');
 	f->store_8('D');
 	f->store_8('S');