perf bpf: Return NULL when RB tree lookup fails in perf_env__find_bpf_prog_info()

perf bpf: Return NULL when RB tree lookup fails in perf_env__find_bpf_prog_info()

We currently don't return NULL in case we don't find the
bpf_prog_info_node, fixing that.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Acked-by: Song Liu <songliubraving@fb.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Fixes: e4378f0cb90b ("perf bpf: Save bpf_prog_info in a rbtree in perf_env")
Link: http://lkml.kernel.org/r/20190416134151.15282-1-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>