Avoid a "No such file or directory" error

Check the file /etc/pihole/pihole-FTL.conf exists before trying to use
it.
without the patch I get, each day, in my logs:
sed: can't read /etc/pihole/pihole-FTL.conf: No such file or directory

I use a variable FTLconf to not repeat the file name twice.

Signed-off-by: Ludovic Rousseau <ludovic.rousseau@free.fr>

advanced/Scripts/piholeLogFlush.sh

@@ -16,7 +16,8 @@ source ${colfile}
 # Constructed to return nothing when
 # a) the setting is not present in the config file, or
 # b) the setting is commented out (e.g. "#DBFILE=...")
-DBFILE="$(sed -n -e 's/^\s*DBFILE\s*=\s*//p' /etc/pihole/pihole-FTL.conf)"
+FTLconf="/etc/pihole/pihole-FTL.conf"
+[ -e "$FTLconf" ] && DBFILE="$(sed -n -e 's/^\s*DBFILE\s*=\s*//p' $FTLconf)"
 # Test for empty string. Use standard path in this case.
 if [ -z "$DBFILE" ]; then
   DBFILE="/etc/pihole/pihole-FTL.db"