tools, bpftool: Fix warning on ignored return value for 'read'

When building bpftool, a warning was introduced by commit a943646036
("bpftool: Allow to read btf as raw data"), because the return value
from a call to 'read()' is ignored. Let's address it.

Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20191119111706.22440-1-quentin.monnet@netronome.com

tools/bpf/bpftool/btf.c

@@ -428,15 +428,15 @@ exit_close:
 static bool is_btf_raw(const char *file)
 {
 	__u16 magic = 0;
-	int fd;
+	int fd, nb_read;
 
 	fd = open(file, O_RDONLY);
 	if (fd < 0)
 		return false;
 
-	read(fd, &magic, sizeof(magic));
+	nb_read = read(fd, &magic, sizeof(magic));
 	close(fd);
-	return magic == BTF_MAGIC;
+	return nb_read == sizeof(magic) && magic == BTF_MAGIC;
 }
 
 static int do_dump(int argc, char **argv)