fixed destructor to detect empty context and avoid segfault
fixed destructor to detect empty context and avoid segfault

polotek/libxmljs,sergeche/libxmljs,gagern/libxmljs,Visya/libxmljs,libxmljs/libxmljs,Konsus/libxml,polotek/libxmljs,hildjj/libxmljs,libxmljs/libxmljs,rc0x03/libxmljs,CodeFridge/libxmljs_mem,isaacs/libxmljs,hildjj/libxmljs,CodeFridge/libxmljs_mem,sergeche/libxmljs,isaacs/libxmljs,improvisio-software/libxmljs,rchipka/libxmljs,improvisio-software/libxmljs,apeace/libxmljs,isaacs/libxmljs,Visya/libxmljs,gagern/libxmljs,rchipka/libxmljs,sergeche/libxmljs,improvisio-software/libxmljs,hildjj/libxmljs,gagern/libxmljs,CodeFridge/libxmljs_mem,rc0x03/libxmljs,rchipka/libxmljs,isaacs/libxmljs,apeace/libxmljs,rc0x03/libxmljs,Konsus/libxml,polotek/libxmljs,libxmljs/libxmljs,Konsus/libxml,Visya/libxmljs