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.