Fix GH-17772: imagepalettetotruecolor segfault on invalid truecolor p… - #17777

Ugh, that's ugly, since it only affects bundled libgd which uses ZendMM; there appear to be no issues with external libgd (which doesn't heed memory_limit anyway). So it's not likely that we can have a fix being applied upstream, but further down the road some sync might get rid of our fix (should probably add a comment, that gdCalloc() needs to stay).

Possibly even worse, if upstream ever implements custom allocators (libgd/libgd#335), in which case such issues (I expect more than this particular one) would also affect them.

The general problem I'm seeing here is that libgd never came around to actually implement a contiguous buffer for the pixels (although that is planned for many, many years), so you would have a single allocation upfront, instead of allocating an individual buffer for each row.