[openwrt/openwrt] build: Fix Shellcheck for get_source_date_epoch.sh

[LEDE Commits]

dangole pushed a commit to openwrt/openwrt.git, branch master:
https://git.openwrt.org/ae87e53e33a7e914ec340bebb45cb177724f5952

commit ae87e53e33a7e914ec340bebb45cb177724f5952
Author: Paul Spooren <mail at aparcar.org>
AuthorDate: Sat Aug 15 10:57:00 2020 -1000

    build: Fix Shellcheck for get_source_date_epoch.sh
    
    If a `cd` to `TOPDIR` fails the script should quit.
    
    Also unify `try_mtime` function by storing it in a variable.
    
    Signed-off-by: Paul Spooren <mail at aparcar.org>
---
 scripts/get_source_date_epoch.sh | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/scripts/get_source_date_epoch.sh b/scripts/get_source_date_epoch.sh
index 84ba445454..8a41eebae7 100755
--- a/scripts/get_source_date_epoch.sh
+++ b/scripts/get_source_date_epoch.sh
@@ -1,7 +1,10 @@
 #!/usr/bin/env bash
 export LANG=C
 export LC_ALL=C
-[ -n "$TOPDIR" ] && cd $TOPDIR
+
+if [ -n "$TOPDIR" ]; then
+	cd "$TOPDIR" || exit 1
+fi
 
 try_version() {
 	[ -f version.date ] || return 1
@@ -22,7 +25,7 @@ try_hg() {
 }
 
 try_mtime() {
-	perl -e 'print((stat $ARGV[0])[9])' "$0"
+	SOURCE_DATE_EPOCH=$(perl -e 'print((stat $ARGV[0])[9])' "$0")
 	[ -n "$SOURCE_DATE_EPOCH" ]
 }