SST script confiugration reader is not defensive

With a typo in the configuration file, the mariabackup SST script is not defensive:

/usr//bin/wsrep_sst_mariabackup: eval: line 128: unexpected EOF while looking for matching `"'

I managed to get this error in the log with these settings in the sst section:
[sst]
open-files-limit=65535
compressor="pigz -1"
decompressor="pigz -d"
inno-apply-opts="--use-memory=1956M"
backup-threads=8"

Note the extra quote at "backup-threads".