coresight: trbe: Return NULL pointer for allocation failures

In the Linux kernel, the following vulnerability has been resolved: coresight: trbe: Return NULL pointer for allocation failures When the TRBE driver fails to allocate a buffer, it currently returns the error code "-ENOMEM". However, the caller etm_setup_aux() only checks for a NULL pointer, so it misses the error. As a result, the driver continues and eventually causes a kernel panic. Fix this by returning a NULL pointer from arm_trbe_alloc_buffer() on allocation failures. This allows that the callers can properly handle the failure.