avfilter/vf_overlay: Remove superfluous ;

avfilter/vf_overlay: Remove superfluous ;

In a function body, a redundant ; is just a null statement that does
nothing. Yet outside a function body, a superfluous ';' like one that
exists if one adds a ';' immediately after a function body's closing
brace is actually invalid C that compilers happen to accept. Yet when
compiled in -pedantic mode, both GCC as well as Clang emit warnings for
this like "ISO C does not allow extra ‘;’ outside of a function
[-Wpedantic]".

The scenario described above existed in vf_overlay.c as a result of
macro expansion. This commit fixes it.

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>