Ensure that return values of calls to qMalloc()/qRealloc() are checked

Ensure that return values of calls to qMalloc()/qRealloc() are checked, there are some cases where it is not checked if 0 is returned.

Note: in most cases, there's no graceful failing (such as memory allocation failure during a detaching of implicitly shared data). It might be necessary to throw an exception, which should result in the application exiting as soon as possible (trying to recover will probably result in leaked memory).