makepkg: ignore epoch when undeclared

In this case, we skip the epoch versioning entirely, as if it were
declared as 0.

Prevents errors such as:

/usr/bin/makepkg: line 244: ((: !  : syntax error: operand expected
(error token is " ")
==> Finished making: cower-git :20110808-1 (Mon Aug  8 17:17:27 EDT
2011)

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

scripts/makepkg.sh.in

@@ -241,7 +241,7 @@ get_url() {
 ##
 get_full_version() {
 	if [[ -z $1 ]]; then
-		if (( ! $epoch )); then
+		if [[ $epoch ]] && (( ! $epoch )); then
 			echo $pkgver-$pkgrel
 		else
 			echo $epoch:$pkgver-$pkgrel