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

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

We don't return NULL when we don't find the bpf_prog_info_node, fix
that.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Reported-by: Song Liu <songliubraving@fb.com>
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: 3792cb2ff43b ("perf bpf: Save BTF in a rbtree in perf_env")
Link: http://lkml.kernel.org/r/20190417145539.11669-1-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>