Don't pass NULL in as a flags argument to libfetch's fetchGetURL()

as this can result in a NULL pointer deference when parsing the
flags later.  This change fixes "pkg_add -r" on 5.0-CURRENT for
me; not quite clear how the problem was introduced.

usr.sbin/pkg_install/lib/url.c

@@ -89,7 +89,7 @@ fileGetURL(const char *base, const char *spec)
     else
 	strcpy(fname, spec);
 
-    if ((ftp = fetchGetURL(fname, Verbose ? "v" : NULL)) == NULL) {
+    if ((ftp = fetchGetURL(fname, Verbose ? "v" : "")) == NULL) {
 	printf("Error: FTP Unable to get %s: %s\n",
 	       fname, fetchLastErrString);
 	return NULL;