Without this hint that bytes is always going to be > 0 we got a million
of warnings like

In function ‘void gfree(void*)’,
    inlined from ‘void* greallocn(void*, int, int, bool, bool)’ at goo/gmem.h:178:14,
    inlined from ‘void CharCodeToUnicode::setMapping(CharCode, Unicode*, int)’ at poppler/CharCodeToUnicode.cc:648:60:
goo/gmem.h:65:14: warning: pointer used after ‘void free(void*)’ [-Wuse-after-free]
   65 |     std::free(p);
      |     ~~~~~~~~~^~~
In function ‘void gfree(void*)’,
    inlined from ‘void* grealloc(void*, size_t, bool)’ at goo/gmem.h:73:14,
    inlined from ‘void* greallocn(void*, int, int, bool, bool)’ at goo/gmem.h:174:27,
    inlined from ‘void CharCodeToUnicode::setMapping(CharCode, Unicode*, int)’ at poppler/CharCodeToUnicode.cc:648:60:
goo/gmem.h:65:14: note: call to ‘void free(void*)’ here
   65 |     std::free(p);
      |     ~~~~~~~~~^~~