perf bpf: Add NULL check for btf__type_by_id() in synthesize_bpf_prog_name()

perf bpf: Add NULL check for btf__type_by_id() in synthesize_bpf_prog_name()

synthesize_bpf_prog_name() calls btf__type_by_id() and immediately
dereferences the result via t->name_off without checking for NULL.
btf__type_by_id() returns NULL when the type_id is invalid or out
of range.  When processing perf.data files, finfo->type_id comes from
untrusted input, so an invalid ID causes a NULL pointer dereference.

Fix by checking t for NULL before dereferencing.

Reported-by: sashiko-bot <sashiko-bot@kernel.org>
Fixes: fc462ac75b36daaa ("perf bpf: Extract logic to create program names from perf_event__synthesize_one_bpf_prog()")
Cc: Song Liu <songliubraving@fb.com>
Assisted-by: Claude:claude-opus-4.6
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>