do not run md5sum if there are nothing to check

abuild

@@ -56,6 +56,9 @@ checkdeps() {
 }
 
 checkmd5() {
+	if [ -z "$source" ]; then
+		return 0
+	fi
 	if [ -z "$md5sums" ]; then
 		die "Use 'abuild checksum >>$APKBUILD' to generate a checksum"
 	fi