cleanup(libscap): remove always-false NULL check in get_source_plugin_stats_v2

[Ququrruzza]

What happened

get_source_plugin_stats_v2 in the source_plugin engine has a NULL check on
stats that can never be true.

userspace/libscap/engine/source_plugin/source_plugin.c (master), around
line 290:

metrics_v2* stats = handle->m_stats;
if(!stats) {                 // always false
    *nstats = 0;
    *rc = SCAP_FAILURE;
    return NULL;
}

m_stats is an array member of the engine struct, not a pointer -
userspace/libscap/engine/source_plugin/source_plugin.h:

metrics_v2 m_stats[MAX_SOURCE_PLUGIN_COUNTERS_STATS];

So stats = handle->m_stats always decays to the address of the first element,
which is non-null whenever handle is valid (and handle is already
dereferenced two lines above without a check). The if(!stats) branch is dead
code.

Flagged by static analysis as a redundant comparison that is always false at
source_plugin.c:291, and confirmed by reading the code.

Why it matters

No functional impact - the branch never runs, so behaviour is unchanged. It's
dead code that trips static analyzers and is slightly misleading (it reads like
a real allocation-failure guard, but m_stats is never allocated or nullable).

Suggested change

Drop the check:

metrics_v2* stats = handle->m_stats;

/* SOURCE PLUGIN STATS COUNTERS */
for(uint32_t stat = 0; stat < MAX_SOURCE_PLUGIN_COUNTERS_STATS; stat++) {
    ...
}

Note

The same metrics_v2* stats = <array member>; if(!stats) pattern likely exists
in other engines' get_stats_v2 implementations, so this could be swept in one
cleanup if wanted.

[Ququrruzza]

/kind cleanup