tzdata: fix postinst

* add quotes around possibly empty tz variable
* use exit instead of return, because we're not in function and postinst
  fails:
  line 9: return: can only `return' from a function or sourced script"

(From OE-Core rev: 87429d37e72b1fd2f329d693b4d1b1229a90e0a2)

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-extended/tzdata/tzdata.inc

@@ -60,8 +60,8 @@ pkg_postinst_${PN} () {
 		tz=$(sed -e 's:#.*::' -e 's:[[:space:]]*::g' -e '/^$/d' "${src}")
 	fi
 	
-	if [ -z ${tz} ] ; then
-		return 0
+	if [ -z "${tz}" ] ; then
+		exit 0
 	fi
 	
 	if [ ! -e "$D${datadir}/zoneinfo/${tz}" ] ; then