FTBFS with GCC 6: statement indented as if it were guarded by...

This package fails to build with GCC 6.

> sbuild (Debian sbuild) 0.67.0 (26 Dec 2015) on dl580gen9-02.hlinux
...
> gcc -DHAVE_CONFIG_H -I. -I.. -I../include/ -Wall -Werror -Wdate-time -D_FORTIFY_SOURCE=2 -g -O2 -fstack-protector-strong -Wformat -Werror=format-security -c -o grail-test-atomic.o grail-test-atomic.c
> grail-test-atomic.c: In function 'process_frame_events':
> grail-test-atomic.c:151:11: error: statement is indented as if it were guarded by... [-Werror=misleading-indentation]
> subscribe(grail_handle, device, subscribe_window, subscriptions, 3,
> ^~~~~~~~~
>
> grail-test-atomic.c:148:9: note: ...this 'else' clause, but it is not
> else
> ^~~~
>