PM: tools: pm-graph: fix ValueError when parsing incomplete device properties

When parsing device properties from ftrace data, the devprops() function
assumes that each line has at least three fields and that the third field
(f[2]) always contains a valid integer. However, due to incomplete or
corrupted ftrace logs, f[2] may be missing, empty, or non-existent.

This can lead to the following error:

    Traceback (most recent call last):
      File "../sleepgraph.py", line 7142, in <module>
        stamp = rerunTest(sysvals.outdir)
      File "../sleepgraph.py", line 6255, in rerunTest
        testruns, stamp = processData()
      File "../sleepgraph.py", line 6181, in processData
        testruns, error = parseTraceLog(live)
      File "../sleepgraph.py", line 3470, in parseTraceLog
        tp, tf = loadTraceLog()
      File "../sleepgraph.py", line 3398, in loadTraceLog
        if tp.stampInfo(line, sysvals):
      File "../sleepgraph.py", line 3073, in stampInfo
        self.parsePlatformInfo(line, sv)
      File "../sleepgraph.py", line 3177, in parsePlatformInfo
        sv.devprops = self.devprops(sv.b64unzip(info))
      File "../sleepgraph.py", line 3158, in devprops
        if int(f[2]):
    ValueError: invalid literal for int() with base 10: ''

To prevent this crash, add proper validation before accessing.

Signed-off-by: Gongwei Li <ligongwei@kylinos.cn>
Acked-by: Todd Brandt <todd.e.brandt@intel.com>
[ rjw: Subject tweak ]
Link: https://patch.msgid.link/20260424071208.3610628-1-13875017792@163.com
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>