Fix potential infinite loop in mix_source_buffer() in case of uneven input buffer

[ivan-mogilko]

Fix #23

This is a hotfix that we've been using in our game engine for a while now, it covers a rare situation with badly formed input buffer, which I've described in the issue #23.

This fixes a potential infinite loop, which may occur in two cases:

1. If remaining data in the input is less than bufferframesize, in such case bytesput resulted in 0, and src->offset never advanced.
2. If remaining data's size is not evenly divided by sizeof(float), the data pointer again will not advance to the end.

[ivan-mogilko]

I've encountered another similar case, just bit lower in code (NOTE this is sdl2 branch):

mojoAL/mojoal.c

Lines 1470 to 1475 in 864c4aa

	const int framesavail = (buffer->len - src->offset) / bufferframesize;
	const int mixframes = SDL_min(framesneeded, framesavail);
	mix_buffer(src, buffer, src->panning, data, *stream, mixframes);
	src->offset += mixframes * bufferframesize;
	*len -= mixframes * deviceframesize;
	*stream += mixframes * ctx->device->channels;

This line
const int framesavail = (buffer->len - src->offset) / bufferframesize;

will halt any progress if a difference between buffer length and offset is less than frame size.

@icculus I realize that the SDL2 branch is in maintenance mode, will you accept a PR with a fix for this problem?
I dont know if SDL3 branch has this issue or not, have not get to build mojoal with SDL3 yet.

[icculus]

I'll take a PR if you're inclined to fix it! Otherwise, I'll try to find some time to look at it.