vshCommandParse: placate coverity

vshCommandParse: placate coverity

There is no real leak here, but Coverity-Prevent thinks there is.
It does not see that while there are four ways to return from
vshCommandGetToken with VSH_TK_END, none of them results in allocation
of a result.
* tools/virsh.c (vshCommandParse): Add a (currently) useless VIR_FREE,
to ensure that we never leak when vshCommandGetToken returns VSH_TK_END.