calculation overflow in grub_mm_init_region

[Leif Lindholm]

When allocating memory for the heap on ARMv7 UEFI, the init code
pretty much just allocates a chunk from the top of available RAM.

This means that when grub_mm_init_region is called for a region
extending to the top of the 32-bit address space, addr + size == 0.
However, this is not taken into account by arithmetic in this
function, causing Grub to fail on one of my platforms[1] before
"Welcome to GRUB".

Having some trouble getting my head around the grub_mm_init_region
code right now (where is grub_mm_base initialised?), so don't have
a patch.

/
    Leif

[1] Versatile Express V2P-CA15_A7, also known as TC2, has 2GB of RAM
    starting at 0x80000000.