fixed segfault with xpath when it returned an empty nodeset
fixed segfault with xpath when it returned an empty nodeset

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