png: fix potential buffer overflow

This is the error we have:

../../../modules/codec/png.c:162:49: warning: '%zu' directive output may be truncated writing between 1 and 20 bytes into a region of size between 18 and 37 [-Wformat-truncation=]
  162 |                   "block size %zu too small for %zu encoded bytes",
      |                                                 ^~~
../../../modules/codec/png.c:162:19: note: directive argument in the range [1, 18446744073709551615]
  162 |                   "block size %zu too small for %zu encoded bytes",