Filter nonexistent directories passed to find

With the `set -e` option, the deb scripts can fail when passing
nonexistent directory to find.

pkg/debian/salt-master.preinst

@@ -23,7 +23,12 @@ case "$1" in
     PY_VER=$(/opt/saltstack/salt/bin/python3 -c "import sys; sys.stdout.write('{}.{}'.format(*sys.version_info)); sys.stdout.flush();")
 
     # Reset permissions to fix previous installs
-    find ${SALT_HOME} /etc/salt /var/log/salt /var/cache/salt /var/run/salt \
+    SALT_DIRS=""
+    for salt_dir in ${SALT_HOME} /etc/salt /var/log/salt /var/cache/salt /var/run/salt
+    do
+        [ -d "${salt_dir}" ] && SALT_DIRS="${SALT_DIRS:+${SALT_DIRS} }${salt_dir}"
+    done
+    find ${SALT_DIRS} \
         \! \( -path /etc/salt/cloud.deploy.d\* -o -path /var/log/salt/cloud -o -path /opt/saltstack/salt/lib/python${PY_VER}/site-packages/salt/cloud/deploy\* \) -a \
         \( -user ${SALT_USER} -o -group ${SALT_GROUP} \) -exec chown root:root \{\} \;