The alloc_size attribute is valid only on functions that return a
pointer.  GCC 9 (not yet released) warns about invalid usage:

./libavutil/mem.h:342:1: warning: 'alloc_size' attribute ignored on a function returning int' [-Wattributes]
  342 | av_alloc_size(2, 3) int av_reallocp_array(void *ptr, size_t nmemb, size_t size);
      | ^~~~~~~~~~~~~
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>