Resolve Coverity complaints in tests/securityselinuxlabeltest.c

Resolve Coverity complaints in tests/securityselinuxlabeltest.c

Two complaints of RESOURCE_FREE due to going to cleanup prior to a
VIR_FREE(line).  Two complaints of FORWARD_NULL due to 'tmp' being
accessed after a strchr() without first checking if the return was NULL.

While looking at the code it seems that 'line' need only be allocated
once as the while loop will keep reading into line until eof causing
an unreported leak since line was never VIR_FREE()'d at the bottom of
the loop.