fix some warnings (and maybe a bug?)

[MLXProjects]

Changes: casted some sizeof() to prevent signed/unsigned comparison moved break; outside the if they were at two switch/case statements, not sure if those fallthroughs were intended tho. at _mxml_vsnprintf, put the char *size variable inside an #ifdef because it's only used if that condition is met

[michaelrsweet]

OK, so most of this code has changed. The only remaining change I'll solve by casting the pointer difference to size_t...

[master 069a353] Fix warning (Issue #301)