Bug 794949 - Plugin crash when opening png, jpeg or tiff with...
... non-latin unicode path. g_win32_locale_filename_from_utf8() was sometimes returning NULL for some paths on Windows. Then the call to gexiv2_metadata_open_path() with a NULL value was crashing plug-ins. This commit only prevents from crashing by simply failing to load metadata when this occurs, which means losing metadata support on Windows depending on filenames. A proper solution will have to be implemented.