trace-cmd: Fix reading of overrun / commit_overrun cpu stat fields

sizeof(overrun_str) includes the nil, so atoi was skipping the lead
digit meaning trace-cmd would report:

CPU 0: 37984 events lost

for "overrun: 137984"

Signed-off-by: Michael Sartain <mikesart@fastmail.com>
[ Use strlen() instead of (sizeof() - 1) ]
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
1 file changed