Properly initialize clone_ts.

Initialize clone_ts when using a stack-residing scap_threadinfo to read
from a proclist. Otherwise, when reading from an old trace file the
initial clone_ts could be invalid, causing this difference in the
proc_exec_time chisel:

diff -r /tmp/sysdig.NzoD98EsyQ/results/proc_exec_time/wordpress.scap.output /tmp/sysdig.NzoD98EsyQ/baseline/proc_exec_time/wordpress.scap.output
3d2
< 1425066765.22 sleep        0.1

userspace/libscap/scap_savefile.c

@@ -881,6 +881,7 @@ static int32_t scap_read_proclist(scap_t *handle, gzFile f, uint32_t block_lengt
 	tinfo.filtered_out = 0;
 	tinfo.root[0] = 0;
 	tinfo.sid = -1;
+	tinfo.clone_ts = 0;
 
 	while(((int32_t)block_length - (int32_t)totreadsize) >= 4)
 	{