MDEV-15512 - Fix sh parse error when [sst] config value has spaces.

Merged https://github.com/MariaDB/server/pull/617 authored by
Colin Mollenhour.

scripts/wsrep_sst_common.sh

@@ -277,8 +277,8 @@ parse_cnf()
     reval=$($MY_PRINT_DEFAULTS "${group}" | awk -v var="${var}" 'BEGIN { OFS=FS="=" } { gsub(/_/,"-",$1); if ( $1=="--"var) lastval=substr($0,length($1)+2) } END { print lastval}')
 
     # use default if we haven't found a value
-    if [ -z $reval ]; then
-        [ -n $3 ] && reval=$3
+    if [ -z "$reval" ]; then
+        [ -n "$3" ] && reval=$3
     fi
     echo $reval
 }