Silence some gcc warnings

cc1: warnings being treated as errors
spice-uri.c: In function ‘spice_uri_parse’:
spice-uri.c:105: error: ‘saveptr’ may be used uninitialized in this
function [-Wuninitialized]
spice-uri.c:105: error: ‘saveptr2’ may be used uninitialized in this
function [-Wuninitialized]