tracing/mmiotrace: Add NULL check for mmio_trace_array in logging functions

In the Linux kernel, the following vulnerability has been resolved: tracing/mmiotrace: Add NULL check for mmio_trace_array in logging functions mmio_trace_rw() and mmio_trace_mapping() retrieve mmio_trace_array into tr and pass it to __trace_mmiotrace_rw() and __trace_mmiotrace_map(). If these functions are invoked while mmio_trace_array is NULL (e.g. before initialization or after disabled), accessing tr->array_buffer.buffer will result in a NULL pointer dereference crash. Fix this by adding an explicit NULL check for tr at the beginning of __trace_mmiotrace_rw() and __trace_mmiotrace_map().