buildrepo: exit early if listing of packages for purging fails

Otherwise we might end up purging more packages than expected if an
APKBUILD has syntax errors

buildrepo.in

@@ -146,7 +146,7 @@ do_build() {
 		local purgefiles
 		cd "$repodir/$repo/$CARCH" || return 1
 		trap 'rm -f "$tmp"; exit 1' INT
-		( listpackages "$1") >$tmp
+		( listpackages "$1") >$tmp || return 1
 		purge=$(ls *.apk 2>/dev/null | grep -v -w -f $tmp)
 		if [ -n "$purge" ]; then
 			rm -f $purge