rts: make `stgMallocBytes` always return non-`NULL`?

Currently stgMallocBytes will bail out when malloc fails, reducing the need to check for NULL on invocation. However, when called with a 0 byte request, it will return NULL.

I propose to slightly change this, and make malloc return a non-NULL pointer even when invoked with 0, similar to the GNU implementation (where malloc(0) is turned into malloc(1), basically). This is within the spec (malloc(0) is guaranteed to return either NULL or a free'able pointer), and would allow to mark stgMallocBytes as __attribute__((returns_nonnull)) which allows for some optimizations to kick in.

(Same could apply to stgReallocBytes and stgCallocBytes, I guess)