Fix signature printing in package info

pm_asprintf() does not return a length as asprintf() does. Fail. Make
sure it is not -1 as that is the only failure condition.

Signed-off-by: Dan McGee <dan@archlinux.org>

src/pacman/util.c

@@ -729,7 +729,7 @@ void signature_display(const char *title, alpm_siglist_t *siglist)
 			name = result->key.uid ? result->key.uid : result->key.fingerprint;
 			ret = pm_asprintf(&sigline, _("%s, %s from \"%s\""),
 					status, validity, name);
-			if(ret < 1) {
+			if(ret == -1) {
 				pm_fprintf(stderr, ALPM_LOG_ERROR,  _("failed to allocate string\n"));
 				continue;
 			}