[Qemu-devel] memory: Add function pointers checks to memory_region_read/write()

["Salva Peiró" <speirofr@gmail.com>]

The situation where QEMU crashes while attempting to call to a NULL
function pointer from a non-initialised field in the MemoryRegionOps
struct happens for the majority of emulated devices:
One approach for solving this is to correct it for each device. 
The other approach is to correct the memory_region_read/write caller
functions at memory.c to ensure that only initialised function pointers
are being called. This approach has the benefit of solving this kind of
error for all emulated devices.

The following patch adds function pointers checks to memory_region_read/write()