serbian: Initialise sbst to 0
serbian: Initialise sbst to 0

This only matters for an input word which contains an 'r' and no
vowels, but we might see such cases from acronyms or typos.

This change probably makes no difference to the output, as all the
suffixes we might remove contains vowels.

snowballstem/snowball,snowballstem/snowball,snowballstem/snowball,snowballstem/snowball,snowballstem/snowball,snowballstem/snowball,snowballstem/snowball,snowballstem/snowball