libbpf: Fix off-by-one error in ELF sanity check

libbpf's bpf_object__elf_collect() does simple sanity check after iterating
over all ELF sections, if checks that .strtab index is correct. Unfortunately,
due to section indices being 1-based, the check breaks for cases when .strtab
ends up being the very last section in ELF.

Fixes: 77ba9a5b ("tools lib bpf: Fetch map names from correct strtab")
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20191028233727.1286699-1-andriin@fb.com

tools/lib/bpf/libbpf.c

@@ -1664,7 +1664,7 @@ static int bpf_object__elf_collect(struct bpf_object *obj, bool relaxed_maps)
 		}
 	}
 
-	if (!obj->efile.strtabidx || obj->efile.strtabidx >= idx) {
+	if (!obj->efile.strtabidx || obj->efile.strtabidx > idx) {
 		pr_warn("Corrupted ELF file: index of strtab invalid\n");
 		return -LIBBPF_ERRNO__FORMAT;
 	}