smyrna: fix incorrect use of 'strtok'

smyrna: fix incorrect use of 'strtok'

This code was using `strtok` as if it splits based on the single separator
passed to it. But `strtok` actually treats the second parameter as a list of
character separators. In this change, we rephrase this code to do what its
original author appears to have intended.

This slightly changes the semantics of this code. But it seems we do not know
the exact intent of the original, so this is hoped to match the author’s
intention.

Gitlab: fixes #2259