fftools/objpool: Don't use return with expression when returning void

Using tail calls with functions returning void is forbidden
(C99/C11 6.8.6.4: "A return statement with an expression shall not appear
in a function whose return type is void.") GCC emits a warning
because of this when using -pedantic: "ISO C forbids ‘return’ with
expression, in function returning void"
Reviewed-by: Hendrik Leppkes <h.leppkes@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

fftools/objpool.c

@@ -101,11 +101,11 @@ static void *alloc_frame(void)
 
 static void reset_packet(void *obj)
 {
-    return av_packet_unref(obj);
+    av_packet_unref(obj);
 }
 static void reset_frame(void *obj)
 {
-    return av_frame_unref(obj);
+    av_frame_unref(obj);
 }
 
 static void free_packet(void **obj)