Fix for SIGSEGV in systemd-bootchart on short-living processes

The function svg_ps_bars() dereferencess NULL pointer in the line
	endtime = ps->last->sampledata->sampletime;
because of partially initialized ps_struct (ps->last == NULL).

If some process terminates between scaning /proc directory in the log_sample()
function and reading additional information from /proc/PID/... files,
the files couldn't be read, the loop will be continued and partially
initialized structure returned.