Crash in qt_memrotate90 or qt_memrotate270

There're occasional crashes in qt_memrotate90 or qt_memrotate270.

The src and dest pointer should be checked before being used. The destination image, created by using malloc, could return 0. memrotate should do nothing if e.g. dest is null.

Unfortunately, there's no sample code to reproduce the crash but
two stackstraces are attached for a better understanding.