buildstats.bbclass: Avoid index exception in /proc/PID/io parsing

buildstats.bbclass: Avoid index exception in /proc/PID/io parsing

There is some probability (depends on system load) to get empty
or line containing "0" as the last line while reading /proc/PID/io.
Avoid build failure by checking if line contains separator
":" before split.

Signed-off-by: Pavel Modilaynen <pavelmn@axis.com>
Signed-off-by: Daniel Lublin <daniel@lublin.se>
Signed-off-by: Ross Burton <ross.burton@intel.com>