Fix minor warnings.

[slembcke]

Fixes some minor warnings that come up with extra warnings enabled.

P.S. This is great and making it basically a drop in replacement for stb_image made it pretty trivial. qoiconv dropped right into my asset pipeline too. :) This shaved dozens of kilobytes from my game, and several milliseconds at load time. Heh. Thanks!

[nigeltao]

An alternative fix would be to change the qoi_decode argument from int size to size_t size. Generally, when passing around a pointer-length pair for a block of memory, the length should be size_t.

Similarly, with these lines:

int size = ftell(f);
...
int bytes_read = fread(data, 1, size, f);

Note that ftell and fread return long and size_t, not int, so there's potential overflow here.

[phoboslab]

Addressed in 81b438c

[nigeltao]

Addressed in 81b438c

Well, this addressed endianness, and changed some of the lines that the PR also changed (so that the PR no longer applies). I filed #21 to discuss the points from #6 (comment) above.