We specify that a "num_hex" comprises 1 or more digits, however, that
allows strtoull to fail with ERANGE. Limit the number of hex digits to
being between 1 and 16.

Before:
```
$ perf stat -e 'cpu/rE7574c47490475745/' true
perf: util/parse-events.c:215: fix_raw: Assertion `errno == 0' failed.
Aborted (core dumped)
```

After:
```
$ perf stat -e 'cpu/rE7574c47490475745/' true
event syntax error: 'cpu/rE7574c47490475745/'
                         \___ Bad event or PMU

Unable to find PMU or event on a PMU of 'cpu'

Initial error:
event syntax error: 'cpu/rE7574c47490475745/'
                         \___ unknown term 'rE7574c47490475745' for pmu 'cpu'

valid terms: event,pc,edge,offcore_rsp,ldlat,inv,umask,frontend,cmask,config,config1,config2,config3,name,period,percore,metric-id
Run 'perf list' for a list of valid events

 Usage: perf stat [<options>] [<command>]

    -e, --event <event>   event selector. use 'perf list' to list available events
```

Issue found through fuzz testing.
Signed-off-by: Ian Rogers <irogers@google.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/r/20230907210533.3712979-1-irogers@google.comSigned-off-by: Namhyung Kim <namhyung@kernel.org>