Fix case when force is specified but jobs not

Signed-off-by: Michal Hrusecky <Michal@Hrusecky.net>

compile_fw

@@ -359,6 +359,7 @@ build() {
 		make $MAKE_DEBUG IS_TTY=1 BUILD_LOG=1 $BUILD_ARGS $OWRT_DEBUG
 	else
 		JOBS="$(echo " $BUILD_ARGS " | sed -n 's|.*-j\([0-9]\+\)|\1\n|p' | head -n 1)"
+		[ -n "$JOBS" ] || JOBS=0
 		SUCCESS=""
 		while [ "$JOBS" -gt 0 ] && [ -z "$SUCCESS" ]; do
 			COUNTDOWN="$FORCE"